assignment with expression

Python Enhancement Proposals

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PEP 572 – Assignment Expressions

The importance of real code, exceptional cases, scope of the target, relative precedence of :=, change to evaluation order, differences between assignment expressions and assignment statements, specification changes during implementation, _pydecimal.py, datetime.py, sysconfig.py, simplifying list comprehensions, capturing condition values, changing the scope rules for comprehensions, alternative spellings, special-casing conditional statements, special-casing comprehensions, lowering operator precedence, allowing commas to the right, always requiring parentheses, why not just turn existing assignment into an expression, with assignment expressions, why bother with assignment statements, why not use a sublocal scope and prevent namespace pollution, style guide recommendations, acknowledgements, a numeric example, appendix b: rough code translations for comprehensions, appendix c: no changes to scope semantics.

This is a proposal for creating a way to assign to variables within an expression using the notation NAME := expr .

As part of this change, there is also an update to dictionary comprehension evaluation order to ensure key expressions are executed before value expressions (allowing the key to be bound to a name and then re-used as part of calculating the corresponding value).

During discussion of this PEP, the operator became informally known as “the walrus operator”. The construct’s formal name is “Assignment Expressions” (as per the PEP title), but they may also be referred to as “Named Expressions” (e.g. the CPython reference implementation uses that name internally).

Naming the result of an expression is an important part of programming, allowing a descriptive name to be used in place of a longer expression, and permitting reuse. Currently, this feature is available only in statement form, making it unavailable in list comprehensions and other expression contexts.

Additionally, naming sub-parts of a large expression can assist an interactive debugger, providing useful display hooks and partial results. Without a way to capture sub-expressions inline, this would require refactoring of the original code; with assignment expressions, this merely requires the insertion of a few name := markers. Removing the need to refactor reduces the likelihood that the code be inadvertently changed as part of debugging (a common cause of Heisenbugs), and is easier to dictate to another programmer.

During the development of this PEP many people (supporters and critics both) have had a tendency to focus on toy examples on the one hand, and on overly complex examples on the other.

The danger of toy examples is twofold: they are often too abstract to make anyone go “ooh, that’s compelling”, and they are easily refuted with “I would never write it that way anyway”.

The danger of overly complex examples is that they provide a convenient strawman for critics of the proposal to shoot down (“that’s obfuscated”).

Yet there is some use for both extremely simple and extremely complex examples: they are helpful to clarify the intended semantics. Therefore, there will be some of each below.

However, in order to be compelling , examples should be rooted in real code, i.e. code that was written without any thought of this PEP, as part of a useful application, however large or small. Tim Peters has been extremely helpful by going over his own personal code repository and picking examples of code he had written that (in his view) would have been clearer if rewritten with (sparing) use of assignment expressions. His conclusion: the current proposal would have allowed a modest but clear improvement in quite a few bits of code.

Another use of real code is to observe indirectly how much value programmers place on compactness. Guido van Rossum searched through a Dropbox code base and discovered some evidence that programmers value writing fewer lines over shorter lines.

Case in point: Guido found several examples where a programmer repeated a subexpression, slowing down the program, in order to save one line of code, e.g. instead of writing:

they would write:

Another example illustrates that programmers sometimes do more work to save an extra level of indentation:

This code tries to match pattern2 even if pattern1 has a match (in which case the match on pattern2 is never used). The more efficient rewrite would have been:

Syntax and semantics

In most contexts where arbitrary Python expressions can be used, a named expression can appear. This is of the form NAME := expr where expr is any valid Python expression other than an unparenthesized tuple, and NAME is an identifier.

The value of such a named expression is the same as the incorporated expression, with the additional side-effect that the target is assigned that value:

There are a few places where assignment expressions are not allowed, in order to avoid ambiguities or user confusion:

This rule is included to simplify the choice for the user between an assignment statement and an assignment expression – there is no syntactic position where both are valid.

Again, this rule is included to avoid two visually similar ways of saying the same thing.

This rule is included to disallow excessively confusing code, and because parsing keyword arguments is complex enough already.

This rule is included to discourage side effects in a position whose exact semantics are already confusing to many users (cf. the common style recommendation against mutable default values), and also to echo the similar prohibition in calls (the previous bullet).

The reasoning here is similar to the two previous cases; this ungrouped assortment of symbols and operators composed of : and = is hard to read correctly.

This allows lambda to always bind less tightly than := ; having a name binding at the top level inside a lambda function is unlikely to be of value, as there is no way to make use of it. In cases where the name will be used more than once, the expression is likely to need parenthesizing anyway, so this prohibition will rarely affect code.

This shows that what looks like an assignment operator in an f-string is not always an assignment operator. The f-string parser uses : to indicate formatting options. To preserve backwards compatibility, assignment operator usage inside of f-strings must be parenthesized. As noted above, this usage of the assignment operator is not recommended.

An assignment expression does not introduce a new scope. In most cases the scope in which the target will be bound is self-explanatory: it is the current scope. If this scope contains a nonlocal or global declaration for the target, the assignment expression honors that. A lambda (being an explicit, if anonymous, function definition) counts as a scope for this purpose.

There is one special case: an assignment expression occurring in a list, set or dict comprehension or in a generator expression (below collectively referred to as “comprehensions”) binds the target in the containing scope, honoring a nonlocal or global declaration for the target in that scope, if one exists. For the purpose of this rule the containing scope of a nested comprehension is the scope that contains the outermost comprehension. A lambda counts as a containing scope.

The motivation for this special case is twofold. First, it allows us to conveniently capture a “witness” for an any() expression, or a counterexample for all() , for example:

Second, it allows a compact way of updating mutable state from a comprehension, for example:

However, an assignment expression target name cannot be the same as a for -target name appearing in any comprehension containing the assignment expression. The latter names are local to the comprehension in which they appear, so it would be contradictory for a contained use of the same name to refer to the scope containing the outermost comprehension instead.

For example, [i := i+1 for i in range(5)] is invalid: the for i part establishes that i is local to the comprehension, but the i := part insists that i is not local to the comprehension. The same reason makes these examples invalid too:

While it’s technically possible to assign consistent semantics to these cases, it’s difficult to determine whether those semantics actually make sense in the absence of real use cases. Accordingly, the reference implementation [1] will ensure that such cases raise SyntaxError , rather than executing with implementation defined behaviour.

This restriction applies even if the assignment expression is never executed:

For the comprehension body (the part before the first “for” keyword) and the filter expression (the part after “if” and before any nested “for”), this restriction applies solely to target names that are also used as iteration variables in the comprehension. Lambda expressions appearing in these positions introduce a new explicit function scope, and hence may use assignment expressions with no additional restrictions.

Due to design constraints in the reference implementation (the symbol table analyser cannot easily detect when names are re-used between the leftmost comprehension iterable expression and the rest of the comprehension), named expressions are disallowed entirely as part of comprehension iterable expressions (the part after each “in”, and before any subsequent “if” or “for” keyword):

A further exception applies when an assignment expression occurs in a comprehension whose containing scope is a class scope. If the rules above were to result in the target being assigned in that class’s scope, the assignment expression is expressly invalid. This case also raises SyntaxError :

(The reason for the latter exception is the implicit function scope created for comprehensions – there is currently no runtime mechanism for a function to refer to a variable in the containing class scope, and we do not want to add such a mechanism. If this issue ever gets resolved this special case may be removed from the specification of assignment expressions. Note that the problem already exists for using a variable defined in the class scope from a comprehension.)

See Appendix B for some examples of how the rules for targets in comprehensions translate to equivalent code.

The := operator groups more tightly than a comma in all syntactic positions where it is legal, but less tightly than all other operators, including or , and , not , and conditional expressions ( A if C else B ). As follows from section “Exceptional cases” above, it is never allowed at the same level as = . In case a different grouping is desired, parentheses should be used.

The := operator may be used directly in a positional function call argument; however it is invalid directly in a keyword argument.

Some examples to clarify what’s technically valid or invalid:

Most of the “valid” examples above are not recommended, since human readers of Python source code who are quickly glancing at some code may miss the distinction. But simple cases are not objectionable:

This PEP recommends always putting spaces around := , similar to PEP 8 ’s recommendation for = when used for assignment, whereas the latter disallows spaces around = used for keyword arguments.)

In order to have precisely defined semantics, the proposal requires evaluation order to be well-defined. This is technically not a new requirement, as function calls may already have side effects. Python already has a rule that subexpressions are generally evaluated from left to right. However, assignment expressions make these side effects more visible, and we propose a single change to the current evaluation order:

• In a dict comprehension {X: Y for ...} , Y is currently evaluated before X . We propose to change this so that X is evaluated before Y . (In a dict display like {X: Y} this is already the case, and also in dict((X, Y) for ...) which should clearly be equivalent to the dict comprehension.)

Most importantly, since := is an expression, it can be used in contexts where statements are illegal, including lambda functions and comprehensions.

Conversely, assignment expressions don’t support the advanced features found in assignment statements:

• Multiple targets are not directly supported: x = y = z = 0 # Equivalent: (z := (y := (x := 0)))
• Single assignment targets other than a single NAME are not supported: # No equivalent a [ i ] = x self . rest = []
• Priority around commas is different: x = 1 , 2 # Sets x to (1, 2) ( x := 1 , 2 ) # Sets x to 1
• Iterable packing and unpacking (both regular or extended forms) are not supported: # Equivalent needs extra parentheses loc = x , y # Use (loc := (x, y)) info = name , phone , * rest # Use (info := (name, phone, *rest)) # No equivalent px , py , pz = position name , phone , email , * other_info = contact
• Inline type annotations are not supported: # Closest equivalent is "p: Optional[int]" as a separate declaration p : Optional [ int ] = None
• Augmented assignment is not supported: total += tax # Equivalent: (total := total + tax)

The following changes have been made based on implementation experience and additional review after the PEP was first accepted and before Python 3.8 was released:

• for consistency with other similar exceptions, and to avoid locking in an exception name that is not necessarily going to improve clarity for end users, the originally proposed TargetScopeError subclass of SyntaxError was dropped in favour of just raising SyntaxError directly. [3]
• due to a limitation in CPython’s symbol table analysis process, the reference implementation raises SyntaxError for all uses of named expressions inside comprehension iterable expressions, rather than only raising them when the named expression target conflicts with one of the iteration variables in the comprehension. This could be revisited given sufficiently compelling examples, but the extra complexity needed to implement the more selective restriction doesn’t seem worthwhile for purely hypothetical use cases.

Examples from the Python standard library

env_base is only used on these lines, putting its assignment on the if moves it as the “header” of the block.

• Current: env_base = os . environ . get ( "PYTHONUSERBASE" , None ) if env_base : return env_base
• Improved: if env_base := os . environ . get ( "PYTHONUSERBASE" , None ): return env_base

Avoid nested if and remove one indentation level.

• Current: if self . _is_special : ans = self . _check_nans ( context = context ) if ans : return ans
• Improved: if self . _is_special and ( ans := self . _check_nans ( context = context )): return ans

Code looks more regular and avoid multiple nested if. (See Appendix A for the origin of this example.)

• Current: reductor = dispatch_table . get ( cls ) if reductor : rv = reductor ( x ) else : reductor = getattr ( x , "__reduce_ex__" , None ) if reductor : rv = reductor ( 4 ) else : reductor = getattr ( x , "__reduce__" , None ) if reductor : rv = reductor () else : raise Error ( "un(deep)copyable object of type %s " % cls )
• Improved: if reductor := dispatch_table . get ( cls ): rv = reductor ( x ) elif reductor := getattr ( x , "__reduce_ex__" , None ): rv = reductor ( 4 ) elif reductor := getattr ( x , "__reduce__" , None ): rv = reductor () else : raise Error ( "un(deep)copyable object of type %s " % cls )

tz is only used for s += tz , moving its assignment inside the if helps to show its scope.

• Current: s = _format_time ( self . _hour , self . _minute , self . _second , self . _microsecond , timespec ) tz = self . _tzstr () if tz : s += tz return s
• Improved: s = _format_time ( self . _hour , self . _minute , self . _second , self . _microsecond , timespec ) if tz := self . _tzstr (): s += tz return s

Calling fp.readline() in the while condition and calling .match() on the if lines make the code more compact without making it harder to understand.

• Current: while True : line = fp . readline () if not line : break m = define_rx . match ( line ) if m : n , v = m . group ( 1 , 2 ) try : v = int ( v ) except ValueError : pass vars [ n ] = v else : m = undef_rx . match ( line ) if m : vars [ m . group ( 1 )] = 0
• Improved: while line := fp . readline (): if m := define_rx . match ( line ): n , v = m . group ( 1 , 2 ) try : v = int ( v ) except ValueError : pass vars [ n ] = v elif m := undef_rx . match ( line ): vars [ m . group ( 1 )] = 0

A list comprehension can map and filter efficiently by capturing the condition:

Similarly, a subexpression can be reused within the main expression, by giving it a name on first use:

Note that in both cases the variable y is bound in the containing scope (i.e. at the same level as results or stuff ).

Assignment expressions can be used to good effect in the header of an if or while statement:

Particularly with the while loop, this can remove the need to have an infinite loop, an assignment, and a condition. It also creates a smooth parallel between a loop which simply uses a function call as its condition, and one which uses that as its condition but also uses the actual value.

An example from the low-level UNIX world:

Rejected alternative proposals

Proposals broadly similar to this one have come up frequently on python-ideas. Below are a number of alternative syntaxes, some of them specific to comprehensions, which have been rejected in favour of the one given above.

A previous version of this PEP proposed subtle changes to the scope rules for comprehensions, to make them more usable in class scope and to unify the scope of the “outermost iterable” and the rest of the comprehension. However, this part of the proposal would have caused backwards incompatibilities, and has been withdrawn so the PEP can focus on assignment expressions.

Broadly the same semantics as the current proposal, but spelled differently.

Since EXPR as NAME already has meaning in import , except and with statements (with different semantics), this would create unnecessary confusion or require special-casing (e.g. to forbid assignment within the headers of these statements).

(Note that with EXPR as VAR does not simply assign the value of EXPR to VAR – it calls EXPR.__enter__() and assigns the result of that to VAR .)

Additional reasons to prefer := over this spelling include:

• In if f(x) as y the assignment target doesn’t jump out at you – it just reads like if f x blah blah and it is too similar visually to if f(x) and y .
• import foo as bar
• except Exc as var
• with ctxmgr() as var

To the contrary, the assignment expression does not belong to the if or while that starts the line, and we intentionally allow assignment expressions in other contexts as well.

• NAME = EXPR
• if NAME := EXPR

reinforces the visual recognition of assignment expressions.

This syntax is inspired by languages such as R and Haskell, and some programmable calculators. (Note that a left-facing arrow y <- f(x) is not possible in Python, as it would be interpreted as less-than and unary minus.) This syntax has a slight advantage over ‘as’ in that it does not conflict with with , except and import , but otherwise is equivalent. But it is entirely unrelated to Python’s other use of -> (function return type annotations), and compared to := (which dates back to Algol-58) it has a much weaker tradition.

This has the advantage that leaked usage can be readily detected, removing some forms of syntactic ambiguity. However, this would be the only place in Python where a variable’s scope is encoded into its name, making refactoring harder.

Execution order is inverted (the indented body is performed first, followed by the “header”). This requires a new keyword, unless an existing keyword is repurposed (most likely with: ). See PEP 3150 for prior discussion on this subject (with the proposed keyword being given: ).

This syntax has fewer conflicts than as does (conflicting only with the raise Exc from Exc notation), but is otherwise comparable to it. Instead of paralleling with expr as target: (which can be useful but can also be confusing), this has no parallels, but is evocative.

One of the most popular use-cases is if and while statements. Instead of a more general solution, this proposal enhances the syntax of these two statements to add a means of capturing the compared value:

This works beautifully if and ONLY if the desired condition is based on the truthiness of the captured value. It is thus effective for specific use-cases (regex matches, socket reads that return '' when done), and completely useless in more complicated cases (e.g. where the condition is f(x) < 0 and you want to capture the value of f(x) ). It also has no benefit to list comprehensions.

Advantages: No syntactic ambiguities. Disadvantages: Answers only a fraction of possible use-cases, even in if / while statements.

Another common use-case is comprehensions (list/set/dict, and genexps). As above, proposals have been made for comprehension-specific solutions.

This brings the subexpression to a location in between the ‘for’ loop and the expression. It introduces an additional language keyword, which creates conflicts. Of the three, where reads the most cleanly, but also has the greatest potential for conflict (e.g. SQLAlchemy and numpy have where methods, as does tkinter.dnd.Icon in the standard library).

As above, but reusing the with keyword. Doesn’t read too badly, and needs no additional language keyword. Is restricted to comprehensions, though, and cannot as easily be transformed into “longhand” for-loop syntax. Has the C problem that an equals sign in an expression can now create a name binding, rather than performing a comparison. Would raise the question of why “with NAME = EXPR:” cannot be used as a statement on its own.

As per option 2, but using as rather than an equals sign. Aligns syntactically with other uses of as for name binding, but a simple transformation to for-loop longhand would create drastically different semantics; the meaning of with inside a comprehension would be completely different from the meaning as a stand-alone statement, while retaining identical syntax.

Regardless of the spelling chosen, this introduces a stark difference between comprehensions and the equivalent unrolled long-hand form of the loop. It is no longer possible to unwrap the loop into statement form without reworking any name bindings. The only keyword that can be repurposed to this task is with , thus giving it sneakily different semantics in a comprehension than in a statement; alternatively, a new keyword is needed, with all the costs therein.

There are two logical precedences for the := operator. Either it should bind as loosely as possible, as does statement-assignment; or it should bind more tightly than comparison operators. Placing its precedence between the comparison and arithmetic operators (to be precise: just lower than bitwise OR) allows most uses inside while and if conditions to be spelled without parentheses, as it is most likely that you wish to capture the value of something, then perform a comparison on it:

Once find() returns -1, the loop terminates. If := binds as loosely as = does, this would capture the result of the comparison (generally either True or False ), which is less useful.

While this behaviour would be convenient in many situations, it is also harder to explain than “the := operator behaves just like the assignment statement”, and as such, the precedence for := has been made as close as possible to that of = (with the exception that it binds tighter than comma).

Some critics have claimed that the assignment expressions should allow unparenthesized tuples on the right, so that these two would be equivalent:

(With the current version of the proposal, the latter would be equivalent to ((point := x), y) .)

However, adopting this stance would logically lead to the conclusion that when used in a function call, assignment expressions also bind less tight than comma, so we’d have the following confusing equivalence:

The less confusing option is to make := bind more tightly than comma.

It’s been proposed to just always require parentheses around an assignment expression. This would resolve many ambiguities, and indeed parentheses will frequently be needed to extract the desired subexpression. But in the following cases the extra parentheses feel redundant:

Frequently Raised Objections

C and its derivatives define the = operator as an expression, rather than a statement as is Python’s way. This allows assignments in more contexts, including contexts where comparisons are more common. The syntactic similarity between if (x == y) and if (x = y) belies their drastically different semantics. Thus this proposal uses := to clarify the distinction.

The two forms have different flexibilities. The := operator can be used inside a larger expression; the = statement can be augmented to += and its friends, can be chained, and can assign to attributes and subscripts.

Previous revisions of this proposal involved sublocal scope (restricted to a single statement), preventing name leakage and namespace pollution. While a definite advantage in a number of situations, this increases complexity in many others, and the costs are not justified by the benefits. In the interests of language simplicity, the name bindings created here are exactly equivalent to any other name bindings, including that usage at class or module scope will create externally-visible names. This is no different from for loops or other constructs, and can be solved the same way: del the name once it is no longer needed, or prefix it with an underscore.

(The author wishes to thank Guido van Rossum and Christoph Groth for their suggestions to move the proposal in this direction. [2] )

As expression assignments can sometimes be used equivalently to statement assignments, the question of which should be preferred will arise. For the benefit of style guides such as PEP 8 , two recommendations are suggested.

• If either assignment statements or assignment expressions can be used, prefer statements; they are a clear declaration of intent.
• If using assignment expressions would lead to ambiguity about execution order, restructure it to use statements instead.

The authors wish to thank Alyssa Coghlan and Steven D’Aprano for their considerable contributions to this proposal, and members of the core-mentorship mailing list for assistance with implementation.

Appendix A: Tim Peters’s findings

Here’s a brief essay Tim Peters wrote on the topic.

I dislike “busy” lines of code, and also dislike putting conceptually unrelated logic on a single line. So, for example, instead of:

instead. So I suspected I’d find few places I’d want to use assignment expressions. I didn’t even consider them for lines already stretching halfway across the screen. In other cases, “unrelated” ruled:

is a vast improvement over the briefer:

The original two statements are doing entirely different conceptual things, and slamming them together is conceptually insane.

In other cases, combining related logic made it harder to understand, such as rewriting:

as the briefer:

The while test there is too subtle, crucially relying on strict left-to-right evaluation in a non-short-circuiting or method-chaining context. My brain isn’t wired that way.

But cases like that were rare. Name binding is very frequent, and “sparse is better than dense” does not mean “almost empty is better than sparse”. For example, I have many functions that return None or 0 to communicate “I have nothing useful to return in this case, but since that’s expected often I’m not going to annoy you with an exception”. This is essentially the same as regular expression search functions returning None when there is no match. So there was lots of code of the form:

I find that clearer, and certainly a bit less typing and pattern-matching reading, as:

It’s also nice to trade away a small amount of horizontal whitespace to get another _line_ of surrounding code on screen. I didn’t give much weight to this at first, but it was so very frequent it added up, and I soon enough became annoyed that I couldn’t actually run the briefer code. That surprised me!

There are other cases where assignment expressions really shine. Rather than pick another from my code, Kirill Balunov gave a lovely example from the standard library’s copy() function in copy.py :

The ever-increasing indentation is semantically misleading: the logic is conceptually flat, “the first test that succeeds wins”:

Using easy assignment expressions allows the visual structure of the code to emphasize the conceptual flatness of the logic; ever-increasing indentation obscured it.

A smaller example from my code delighted me, both allowing to put inherently related logic in a single line, and allowing to remove an annoying “artificial” indentation level:

That if is about as long as I want my lines to get, but remains easy to follow.

So, in all, in most lines binding a name, I wouldn’t use assignment expressions, but because that construct is so very frequent, that leaves many places I would. In most of the latter, I found a small win that adds up due to how often it occurs, and in the rest I found a moderate to major win. I’d certainly use it more often than ternary if , but significantly less often than augmented assignment.

I have another example that quite impressed me at the time.

Where all variables are positive integers, and a is at least as large as the n’th root of x, this algorithm returns the floor of the n’th root of x (and roughly doubling the number of accurate bits per iteration):

It’s not obvious why that works, but is no more obvious in the “loop and a half” form. It’s hard to prove correctness without building on the right insight (the “arithmetic mean - geometric mean inequality”), and knowing some non-trivial things about how nested floor functions behave. That is, the challenges are in the math, not really in the coding.

If you do know all that, then the assignment-expression form is easily read as “while the current guess is too large, get a smaller guess”, where the “too large?” test and the new guess share an expensive sub-expression.

To my eyes, the original form is harder to understand:

This appendix attempts to clarify (though not specify) the rules when a target occurs in a comprehension or in a generator expression. For a number of illustrative examples we show the original code, containing a comprehension, and the translation, where the comprehension has been replaced by an equivalent generator function plus some scaffolding.

Since [x for ...] is equivalent to list(x for ...) these examples all use list comprehensions without loss of generality. And since these examples are meant to clarify edge cases of the rules, they aren’t trying to look like real code.

Note: comprehensions are already implemented via synthesizing nested generator functions like those in this appendix. The new part is adding appropriate declarations to establish the intended scope of assignment expression targets (the same scope they resolve to as if the assignment were performed in the block containing the outermost comprehension). For type inference purposes, these illustrative expansions do not imply that assignment expression targets are always Optional (but they do indicate the target binding scope).

Let’s start with a reminder of what code is generated for a generator expression without assignment expression.

• Original code (EXPR usually references VAR): def f (): a = [ EXPR for VAR in ITERABLE ]
• Translation (let’s not worry about name conflicts): def f (): def genexpr ( iterator ): for VAR in iterator : yield EXPR a = list ( genexpr ( iter ( ITERABLE )))

Let’s add a simple assignment expression.

• Original code: def f (): a = [ TARGET := EXPR for VAR in ITERABLE ]
• Translation: def f (): if False : TARGET = None # Dead code to ensure TARGET is a local variable def genexpr ( iterator ): nonlocal TARGET for VAR in iterator : TARGET = EXPR yield TARGET a = list ( genexpr ( iter ( ITERABLE )))

Let’s add a global TARGET declaration in f() .

• Original code: def f (): global TARGET a = [ TARGET := EXPR for VAR in ITERABLE ]
• Translation: def f (): global TARGET def genexpr ( iterator ): global TARGET for VAR in iterator : TARGET = EXPR yield TARGET a = list ( genexpr ( iter ( ITERABLE )))

Or instead let’s add a nonlocal TARGET declaration in f() .

• Original code: def g (): TARGET = ... def f (): nonlocal TARGET a = [ TARGET := EXPR for VAR in ITERABLE ]
• Translation: def g (): TARGET = ... def f (): nonlocal TARGET def genexpr ( iterator ): nonlocal TARGET for VAR in iterator : TARGET = EXPR yield TARGET a = list ( genexpr ( iter ( ITERABLE )))

Finally, let’s nest two comprehensions.

• Original code: def f (): a = [[ TARGET := i for i in range ( 3 )] for j in range ( 2 )] # I.e., a = [[0, 1, 2], [0, 1, 2]] print ( TARGET ) # prints 2
• Translation: def f (): if False : TARGET = None def outer_genexpr ( outer_iterator ): nonlocal TARGET def inner_generator ( inner_iterator ): nonlocal TARGET for i in inner_iterator : TARGET = i yield i for j in outer_iterator : yield list ( inner_generator ( range ( 3 ))) a = list ( outer_genexpr ( range ( 2 ))) print ( TARGET )

Because it has been a point of confusion, note that nothing about Python’s scoping semantics is changed. Function-local scopes continue to be resolved at compile time, and to have indefinite temporal extent at run time (“full closures”). Example:

This document has been placed in the public domain.

Source: https://github.com/python/peps/blob/main/peps/pep-0572.rst

Last modified: 2023-10-11 12:05:51 GMT

Learning Python by doing

• suggest edit

Variables, Expressions, and Assignments

Variables, expressions, and assignments 1 #, introduction #.

In this chapter, we introduce some of the main building blocks needed to create programs–that is, variables, expressions, and assignments. Programming related variables can be intepret in the same way that we interpret mathematical variables, as elements that store values that can later be changed. Usually, variables and values are used within the so-called expressions. Once again, just as in mathematics, an expression is a construct of values and variables connected with operators that result in a new value. Lastly, an assignment is a language construct know as an statement that assign a value (either as a constant or expression) to a variable. The rest of this notebook will dive into the main concepts that we need to fully understand these three language constructs.

Values and Types #

A value is the basic unit used in a program. It may be, for instance, a number respresenting temperature. It may be a string representing a word. Some values are 42, 42.0, and ‘Hello, Data Scientists!’.

Each value has its own type : 42 is an integer ( int in Python), 42.0 is a floating-point number ( float in Python), and ‘Hello, Data Scientists!’ is a string ( str in Python).

The Python interpreter can tell you the type of a value: the function type takes a value as argument and returns its corresponding type.

Observe the difference between type(42) and type('42') !

Expressions and Statements #

On the one hand, an expression is a combination of values, variables, and operators.

A value all by itself is considered an expression, and so is a variable.

When you type an expression at the prompt, the interpreter evaluates it, which means that it calculates the value of the expression and displays it.

In boxes above, m has the value 27 and m + 25 has the value 52 . m + 25 is said to be an expression.

On the other hand, a statement is an instruction that has an effect, like creating a variable or displaying a value.

The first statement initializes the variable n with the value 17 , this is a so-called assignment statement .

The second statement is a print statement that prints the value of the variable n .

The effect is not always visible. Assigning a value to a variable is not visible, but printing the value of a variable is.

Assignment Statements #

We have already seen that Python allows you to evaluate expressions, for instance 40 + 2 . It is very convenient if we are able to store the calculated value in some variable for future use. The latter can be done via an assignment statement. An assignment statement creates a new variable with a given name and assigns it a value.

The example in the previous code contains three assignments. The first one assigns the value of the expression 40 + 2 to a new variable called magicnumber ; the second one assigns the value of π to the variable pi , and; the last assignment assigns the string value 'Data is eatig the world' to the variable message .

Programmers generally choose names for their variables that are meaningful. In this way, they document what the variable is used for.

Do It Yourself!

Let’s compute the volume of a cube with side \(s = 5\) . Remember that the volume of a cube is defined as \(v = s^3\) . Assign the value to a variable called volume .

Well done! Now, why don’t you print the result in a message? It can say something like “The volume of the cube with side 5 is \(volume\) ”.

Beware that there is no checking of types ( type checking ) in Python, so a variable to which you have assigned an integer may be re-used as a float, even if we provide type-hints .

Names and Keywords #

Names of variable and other language constructs such as functions (we will cover this topic later), should be meaningful and reflect the purpose of the construct.

In general, Python names should adhere to the following rules:

It should start with a letter or underscore.

It cannot start with a number.

It must only contain alpha-numeric (i.e., letters a-z A-Z and digits 0-9) characters and underscores.

They cannot share the name of a Python keyword.

If you use illegal variable names you will get a syntax error.

By choosing the right variables names you make the code self-documenting, what is better the variable v or velocity ?

The following are examples of invalid variable names.

These basic development principles are sometimes called architectural rules . By defining and agreeing upon architectural rules you make it easier for you and your fellow developers to understand and modify your code.

If you want to read more on this, please have a look at Code complete a book by Steven McConnell [ McC04 ] .

Every programming language has a collection of reserved keywords . They are used in predefined language constructs, such as loops and conditionals . These language concepts and their usage will be explained later.

The interpreter uses keywords to recognize these language constructs in a program. Python 3 has the following keywords:

False class finally is return

None continue for lambda try

True def from nonlocal while

and del global not with

as elif if or yield

assert else import pass break

except in raise

Reassignments #

It is allowed to assign a new value to an existing variable. This process is called reassignment . As soon as you assign a value to a variable, the old value is lost.

The assignment of a variable to another variable, for instance b = a does not imply that if a is reassigned then b changes as well.

You have a variable salary that shows the weekly salary of an employee. However, you want to compute the monthly salary. Can you reassign the value to the salary variable according to the instruction?

Updating Variables #

A frequently used reassignment is for updating puposes: the value of a variable depends on the previous value of the variable.

This statement expresses “get the current value of x , add one, and then update x with the new value.”

Beware, that the variable should be initialized first, usually with a simple assignment.

Do you remember the salary excercise of the previous section (cf. 13. Reassignments)? Well, if you have not done it yet, update the salary variable by using its previous value.

Updating a variable by adding 1 is called an increment ; subtracting 1 is called a decrement . A shorthand way of doing is using += and -= , which stands for x = x + ... and x = x - ... respectively.

Order of Operations #

Expressions may contain multiple operators. The order of evaluation depends on the priorities of the operators also known as rules of precedence .

For mathematical operators, Python follows mathematical convention. The acronym PEMDAS is a useful way to remember the rules:

Parentheses have the highest precedence and can be used to force an expression to evaluate in the order you want. Since expressions in parentheses are evaluated first, 2 * (3 - 1) is 4 , and (1 + 1)**(5 - 2) is 8 . You can also use parentheses to make an expression easier to read, even if it does not change the result.

Exponentiation has the next highest precedence, so 1 + 2**3 is 9 , not 27 , and 2 * 3**2 is 18 , not 36 .

Multiplication and division have higher precedence than addition and subtraction . So 2 * 3 - 1 is 5 , not 4 , and 6 + 4 / 2 is 8 , not 5 .

Operators with the same precedence are evaluated from left to right (except exponentiation). So in the expression degrees / 2 * pi , the division happens first and the result is multiplied by pi . To divide by 2π, you can use parentheses or write: degrees / 2 / pi .

In case of doubt, use parentheses!

Let’s see what happens when we evaluate the following expressions. Just run the cell to check the resulting value.

Floor Division and Modulus Operators #

The floor division operator // divides two numbers and rounds down to an integer.

For example, suppose that driving to the south of France takes 555 minutes. You might want to know how long that is in hours.

Conventional division returns a floating-point number.

Hours are normally not represented with decimal points. Floor division returns the integer number of hours, dropping the fraction part.

You spend around 225 minutes every week on programming activities. You want to know around how many hours you invest to this activity during a month. Use the \(//\) operator to give the answer.

The modulus operator % works on integer values. It computes the remainder when dividing the first integer by the second one.

The modulus operator is more useful than it seems.

For example, you can check whether one number is divisible by another—if x % y is zero, then x is divisible by y .

String Operations #

In general, you cannot perform mathematical operations on strings, even if the strings look like numbers, so the following operations are illegal: '2'-'1' 'eggs'/'easy' 'third'*'a charm'

But there are two exceptions, + and * .

The + operator performs string concatenation, which means it joins the strings by linking them end-to-end.

The * operator also works on strings; it performs repetition.

Speedy Gonzales is a cartoon known to be the fastest mouse in all Mexico . He is also famous for saying “Arriba Arriba Andale Arriba Arriba Yepa”. Can you use the following variables, namely arriba , andale and yepa to print the mentioned expression? Don’t forget to use the string operators.

Asking the User for Input #

The programs we have written so far accept no input from the user.

To get data from the user through the Python prompt, we can use the built-in function input .

When input is called your whole program stops and waits for the user to enter the required data. Once the user types the value and presses Return or Enter , the function returns the input value as a string and the program continues with its execution.

Try it out!

You can also print a message to clarify the purpose of the required input as follows.

The resulting string can later be translated to a different type, like an integer or a float. To do so, you use the functions int and float , respectively. But be careful, the user might introduce a value that cannot be converted to the type you required.

We want to know the name of a user so we can display a welcome message in our program. The message should say something like “Hello \(name\) , welcome to our hello world program!”.

Script Mode #

So far we have run Python in interactive mode in these Jupyter notebooks, which means that you interact directly with the interpreter in the code cells . The interactive mode is a good way to get started, but if you are working with more than a few lines of code, it can be clumsy. The alternative is to save code in a file called a script and then run the interpreter in script mode to execute the script. By convention, Python scripts have names that end with .py .

Use the PyCharm icon in Anaconda Navigator to create and execute stand-alone Python scripts. Later in the course, you will have to work with Python projects for the assignments, in order to get acquainted with another way of interacing with Python code.

This Jupyter Notebook is based on Chapter 2 of the books Python for Everybody [ Sev16 ] and Think Python (Sections 5.1, 7.1, 7.2, and 5.12) [ Dow15 ] .

Python 101 – Assignment Expressions

Assignment expressions were added to Python in version 3.8 . The general idea is that an assignment expression allows you to assign to variables within an expression.

The syntax for doing this is:

This operator has been called the “walrus operator”, although their real name is “assignment expression”. Interestingly, the CPython internals also refer to them as “named expressions”.

You can read all about assignment expressions in PEP 572 . Let’s find out how to use assignment expressions!

Using Assignment Expressions

Assignment expressions are still relatively rare. However, you need to know about assignment expressions because you will probably come across them from time to time. PEP 572 has some good examples of assignment expressions.

In these 3 examples, you are creating a variable in the expression statement itself. The first example creates the variable match by assigning it the result of the regex pattern search. The second example assigns the variable value to the result of calling a function in the while loop’s expression. Finally, you assign the result of calling f(x) to the variable y inside of a list comprehension.

It would probably help to see the difference between code that doesn’t use an assignment expression and one that does. Here’s an example of reading a file in chunks:

This code will open up a file of indeterminate size and process it 1024 bytes at a time. You will find this useful when working with very large files as it prevents you from loading the entire file into memory. If you do, you can run out of memory and cause your application or even the computer to crash.

You can shorten this code up a bit by using an assignment expression:

Here you assign the result of the read() to data within the while loop’s expression. This allows you to then use that variable inside of the while loop’s code block. It also checks that some data was returned so you don’t have to have the if not data: break stanza.

Another good example that is mentioned in PEP 572 is taken from Python’s own site.py . Here’s how the code was originally:

And this is how it could be simplified by using an assignment expression:

You move the assignment into the conditional statement’s expression, which shortens the code up nicely.

Now let’s discover some of the situations where assignment expressions can’t be used.

What You Cannot Do With Assignment Expressions

There are several cases where assignment expressions cannot be used.

One of the most interesting features of assignment expressions is that they can be used in contexts that an assignment statement cannot, such as in a lambda or the previously mentioned comprehension. However, they do NOT support some things that assignment statements can do. For example, you cannot do multiple target assignment:

Another prohibited use case is using an assignment expression at the top level of an expression statement. Here is an example from PEP 572:

There is a detailed list of other cases where assignment expressions are prohibited or discouraged in the PEP. You should check that document out if you plan to use assignment expressions often.

Wrapping Up

Assignment expressions are an elegant way to clean up certain parts of your code. The feature’s syntax is kind of similar to type hinting a variable. Once you have the hang of one, the other should become easier to do as well.

In this article, you saw some real-world examples of using the “walrus operator”. You also learned when assignment expressions shouldn’t be used. This syntax is only available in Python 3.8 or newer, so if you happen to be forced to use an older version of Python, this feature won’t be of much use to you.

Related Reading

This article is based on a chapter from Python 101, 2nd Edition , which you can purchase on Leanpub or Amazon .

If you’d like to learn more Python, then check out these tutorials:

Python 101 – How to Work with Images

Python 101 – Documenting Your Code

Python 101: An Intro to Working with JSON

Python 101 – Creating Multiple Processes

5. Assignment Expressions

By Bernd Klein . Last modified: 30 Nov 2023.

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Introduction

This section in our Python tutorial explores the "assignment expression operator" affectionately known to many as "the walrus operator. So, you can call it either the "walrus operator" or the "assignment expression operator," and people in the Python programming community will generally understand what you mean. Despite being introduced in Python version 3.8, we can still view it as a relatively recent addition to the language. The assignment expressions have been discussed in PEP 572 and this is what was written about the naming:

The purpose of this feature is not a new way to assign objects to variables, but it gives programmers a convenient way to assign variables in the middle of expressions.

You might still be curious about the origin of the term "walrus operator." It's affectionately named this way because, with a touch of imagination, the operator's characters resemble the eyes and tusks of a walrus.

We will introduce the assignment expression by comparing it first with a simple assignment statement. A simple assignment statement can also be replaced by an assignment expression, even though it looks clumsy and is definitely not the intended use case of it:

First you may wonder about the brackets, when you look at the assignment expression. It is not meant as a replacement for the simple "assignment statement". Its primary role is to be utilized within expressions

The following code shows a proper usecase:

Though you may argue that the following code might be even clearer:

Here's another Python code example that demonstrates a similar scenario. First with the walrus operator:

Now without the walrus operator:

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Beneficial applications

Now, let's explore some examples where the usage of the assignment expression is really beneficial compared to code not using it.

You can observe that if we choose to avoid using the assignment expression in the list comprehension, we would need to call the function twice. So using the assgment expression makes the code in this context more efficient.

Use Case: Regular Expressions

There is also a great advantage when we use regular expressions. Like in our previous examples we present again two similiar code snippets, the first one without and the second one with the walrus operator:

Now, the code using assignment expressions:

Assignment Expression in File Reading

Fixed-width files.

Files in which each line has the same length are often referred to as "fixed-width" or "fixed-length" files. In these files, each line (or record) is structured so that it contains a predefined number of characters, and the fields within the lines have fixed positions. The files can be read by using the read -method and the walrus operator:

Now the same in traditional coding style:

The benefits for the most recent code snippet are

• It uses a more traditional coding style, which may be more familiar to some developers.
• It assigns and uses the data variable explicitly, making the code easier to understand for those not familiar with assignment expressions.

However, a significant drawback of this approach is that it requires the explicit assignment and reassignment of the data variable, which can be seen as less concise and somewhat less elegant.

Using readline

Most people use a for loop to iterator over a text file line by line. With the walrus operator, we can also elegantly go through a text using the method readline :

Code snippet using readline but not the assignment expression:

Now with the walrus operator:

Another Use Case

In the chapter on while loops of our Python Tutorial, we had a little number guessing game:

As you can see, we had to initialize guess to zero to be able to enter the loop. We can do the initialization directly in the loop condition with an assignment expression and simplify the whole code by this:

Critiques of the Walrus Operator in Python

We said in the beginning of this page that some Python programmers longed for this construct for quite a while. One reason why it was not introduced earlier was the fact that it can also be used to write code which is less readable. In fact, the application of the walrus operator contradicts several principles highlighted in the Zen of Python. To grasp these contraventions, let's delve into the ensuing scenarios.

The Zen of Python says "Explicit is better than implicit". The walrus operator violates this requirement of the Zen of Python. This means that explicit operations are always better than implicit operations. The walrus operator assigns a value to a variable and implicitly returns the value as well. Therefore, it does not align with the concept of "Explicit is better than implicit."

The following code snippet is shows an extreme example which is not recommended to use:

What are your thoughts on the following piece of code using the Python assignment operator inside of a print function call?

These Python code snipets certainly violate two other demands of the Zen of Python:

• Beautiful is Better Than Ugly
• Complex is Better Than Complicated

The principle "There should be one and preferably only one obvious way to do it" from the Zen of Python emphasizes the importance of having a clear and singular approach. When we have the option to separate assignment and return operations into distinct statements, opting for the less readable and more complex walrus operator contradicts this principle.

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Python Conditional Assignment

When you want to assign a value to a variable based on some condition, like if the condition is true then assign a value to the variable, else assign some other value to the variable, then you can use the conditional assignment operator.

In this tutorial, we will look at different ways to assign values to a variable based on some condition.

1. Using Ternary Operator

The ternary operator is very special operator in Python, it is used to assign a value to a variable based on some condition.

It goes like this:

Here, the value of variable will be value_if_true if the condition is true, else it will be value_if_false .

Let's see a code snippet to understand it better.

You can see we have conditionally assigned a value to variable c based on the condition a > b .

2. Using if-else statement

if-else statements are the core part of any programming language, they are used to execute a block of code based on some condition.

Using an if-else statement, we can assign a value to a variable based on the condition we provide.

Here is an example of replacing the above code snippet with the if-else statement.

3. Using Logical Short Circuit Evaluation

Logical short circuit evaluation is another way using which you can assign a value to a variable conditionally.

The format of logical short circuit evaluation is:

It looks similar to ternary operator, but it is not. Here the condition and value_if_true performs logical AND operation, if both are true then the value of variable will be value_if_true , or else it will be value_if_false .

Let's see an example:

But if we make condition True but value_if_true False (or 0 or None), then the value of variable will be value_if_false .

So, you can see that the value of c is 20 even though the condition a < b is True .

So, you should be careful while using logical short circuit evaluation.

While working with lists , we often need to check if a list is empty or not, and if it is empty then we need to assign some default value to it.

Let's see how we can do it using conditional assignment.

Here, we have assigned a default value to my_list if it is empty.

Assign a value to a variable conditionally based on the presence of an element in a list.

Now you know 3 different ways to assign a value to a variable conditionally. Any of these methods can be used to assign a value when there is a condition.

The cleanest and fastest way to conditional value assignment is the ternary operator .

if-else statement is recommended to use when you have to execute a block of code based on some condition.

Happy coding! 😊

Operators and Expressions in Python

Table of Contents

Getting Started With Operators and Expressions

The assignment operator and statements, arithmetic operators and expressions in python, comparison of integer values, comparison of floating-point values, comparison of strings, comparison of lists and tuples, boolean expressions involving boolean operands, evaluation of regular objects in a boolean context, boolean expressions involving other types of operands, compound logical expressions and short-circuit evaluation, idioms that exploit short-circuit evaluation, compound vs chained expressions, conditional expressions or the ternary operator, identity operators and expressions in python, membership operators and expressions in python, concatenation and repetition operators and expressions, the walrus operator and assignment expressions, bitwise operators and expressions in python, operator precedence in python, augmented assignment operators and expressions.

In Python, operators are special symbols, combinations of symbols, or keywords that designate some type of computation. You can combine objects and operators to build expressions that perform the actual computation. So, operators are the building blocks of expressions, which you can use to manipulate your data. Therefore, understanding how operators work in Python is essential for you as a programmer.

In this tutorial, you’ll learn about the operators that Python currently supports. You’ll also learn the basics of how to use these operators to build expressions.

In this tutorial, you’ll:

• Get to know Python’s arithmetic operators and use them to build arithmetic expressions
• Explore Python’s comparison , Boolean , identity , and membership operators
• Build expressions with comparison, Boolean, identity, and membership operators
• Learn about Python’s bitwise operators and how to use them
• Combine and repeat sequences using the concatenation and repetition operators
• Understand the augmented assignment operators and how they work

To get the most out of this tutorial, you should have a basic understanding of Python programming concepts, such as variables , assignments , and built-in data types .

Free Bonus: Click here to download your comprehensive cheat sheet covering the various operators in Python.

Take the Quiz: Test your knowledge with our interactive “Python Operators and Expressions” quiz. You’ll receive a score upon completion to help you track your learning progress:

Interactive Quiz

Test your understanding of Python operators and expressions.

In programming, an operator is usually a symbol or combination of symbols that allows you to perform a specific operation. This operation can act on one or more operands . If the operation involves a single operand, then the operator is unary . If the operator involves two operands, then the operator is binary .

For example, in Python, you can use the minus sign ( - ) as a unary operator to declare a negative number. You can also use it to subtract two numbers:

In this code snippet, the minus sign ( - ) in the first example is a unary operator, and the number 273.15 is the operand. In the second example, the same symbol is a binary operator, and the numbers 5 and 2 are its left and right operands.

Programming languages typically have operators built in as part of their syntax. In many languages, including Python, you can also create your own operator or modify the behavior of existing ones, which is a powerful and advanced feature to have.

In practice, operators provide a quick shortcut for you to manipulate data, perform mathematical calculations, compare values, run Boolean tests, assign values to variables, and more. In Python, an operator may be a symbol, a combination of symbols, or a keyword , depending on the type of operator that you’re dealing with.

For example, you’ve already seen the subtraction operator, which is represented with a single minus sign ( - ). The equality operator is a double equal sign ( == ). So, it’s a combination of symbols:

In this example, you use the Python equality operator ( == ) to compare two numbers. As a result, you get True , which is one of Python’s Boolean values.

Speaking of Boolean values, the Boolean or logical operators in Python are keywords rather than signs, as you’ll learn in the section about Boolean operators and expressions . So, instead of the odd signs like || , && , and ! that many other programming languages use, Python uses or , and , and not .

Using keywords instead of odd signs is a really cool design decision that’s consistent with the fact that Python loves and encourages code’s readability .

You’ll find several categories or groups of operators in Python. Here’s a quick list of those categories:

• Assignment operators
• Arithmetic operators
• Comparison operators
• Boolean or logical operators
• Identity operators
• Membership operators
• Concatenation and repetition operators
• Bitwise operators

All these types of operators take care of specific types of computations and data-processing tasks. You’ll learn more about these categories throughout this tutorial. However, before jumping into more practical discussions, you need to know that the most elementary goal of an operator is to be part of an expression . Operators by themselves don’t do much:

As you can see in this code snippet, if you use an operator without the required operands, then you’ll get a syntax error . So, operators must be part of expressions, which you can build using Python objects as operands.

So, what is an expression anyway? Python has simple and compound statements. A simple statement is a construct that occupies a single logical line , like an assignment statement. A compound statement is a construct that occupies multiple logical lines, such as a for loop or a conditional statement. An expression is a simple statement that produces and returns a value.

You’ll find operators in many expressions. Here are a few examples:

In the first two examples, you use the addition and division operators to construct two arithmetic expressions whose operands are integer numbers. In the last example, you use the equality operator to create a comparison expression. In all cases, you get a specific value after executing the expression.

Note that not all expressions use operators. For example, a bare function call is an expression that doesn’t require any operator:

In the first example, you call the built-in abs() function to get the absolute value of -7 . Then, you compute 2 to the power of 8 using the built-in pow() function. These function calls occupy a single logical line and return a value. So, they’re expressions.

Finally, the call to the built-in print() function is another expression. This time, the function doesn’t return a fruitful value, but it still returns None , which is the Python null type . So, the call is technically an expression.

Note: All Python functions have a return value, either explicit or implicit. If you don’t provide an explicit return statement when defining a function, then Python will automatically make the function return None .

Even though all expressions are statements, not all statements are expressions. For example, pure assignment statements don’t return any value, as you’ll learn in a moment. Therefore, they’re not expressions. The assignment operator is a special operator that doesn’t create an expression but a statement.

Note: Since version 3.8, Python also has what it calls assignment expressions. These are special types of assignments that do return a value. You’ll learn more about this topic in the section The Walrus Operator and Assignment Expressions .

Okay! That was a quick introduction to operators and expressions in Python. Now it’s time to dive deeper into the topic. To kick things off, you’ll start with the assignment operator and statements.

The assignment operator is one of the most frequently used operators in Python. The operator consists of a single equal sign ( = ), and it operates on two operands. The left-hand operand is typically a variable , while the right-hand operand is an expression.

Note: As you already learned, the assignment operator doesn’t create an expression. Instead, it creates a statement that doesn’t return any value.

The assignment operator allows you to assign values to variables . Strictly speaking, in Python, this operator makes variables or names refer to specific objects in your computer’s memory. In other words, an assignment creates a reference to a concrete object and attaches that reference to the target variable.

Note: To dive deeper into using the assignment operator, check out Python’s Assignment Operator: Write Robust Assignments .

For example, all the statements below create new variables that hold references to specific objects:

In the first statement, you create the number variable, which holds a reference to the number 42 in your computer’s memory. You can also say that the name number points to 42 , which is a concrete object.

In the rest of the examples, you create other variables that point to other types of objects, such as a string , tuple , and list , respectively.

You’ll use the assignment operator in many of the examples that you’ll write throughout this tutorial. More importantly, you’ll use this operator many times in your own code. It’ll be your forever friend. Now you can dive into other Python operators!

Arithmetic operators are those operators that allow you to perform arithmetic operations on numeric values. Yes, they come from math, and in most cases, you’ll represent them with the usual math signs. The following table lists the arithmetic operators that Python currently supports:

Note that a and b in the Sample Expression column represent numeric values, such as integer , floating-point , complex , rational , and decimal numbers.

Here are some examples of these operators in use:

In this code snippet, you first create two new variables, a and b , holding 5 and 2 , respectively. Then you use these variables to create different arithmetic expressions using a specific operator in each expression.

Note: The Python REPL will display the return value of an expression as a way to provide immediate feedback to you. So, when you’re in an interactive session, you don’t need to use the print() function to check the result of an expression. You can just type in the expression and press Enter to get the result.

Again, the standard division operator ( / ) always returns a floating-point number, even if the dividend is evenly divisible by the divisor:

In the first example, 10 is evenly divisible by 5 . Therefore, this operation could return the integer 2 . However, it returns the floating-point number 2.0 . In the second example, 10.0 is a floating-point number, and 5 is an integer. In this case, Python internally promotes 5 to 5.0 and runs the division. The result is a floating-point number too.

Note: With complex numbers, the division operator doesn’t return a floating-point number but a complex one:

Here, you run a division between an integer and a complex number. In this case, the standard division operator returns a complex number.

Finally, consider the following examples of using the floor division ( // ) operator:

Floor division always rounds down . This means that the result is the greatest integer that’s smaller than or equal to the quotient. For positive numbers, it’s as though the fractional portion is truncated, leaving only the integer portion.

Comparison Operators and Expressions in Python

The Python comparison operators allow you to compare numerical values and any other objects that support them. The table below lists all the currently available comparison operators in Python:

The comparison operators are all binary. This means that they require left and right operands. These operators always return a Boolean value ( True or False ) that depends on the truth value of the comparison at hand.

Note that comparisons between objects of different data types often don’t make sense and sometimes aren’t allowed in Python. For example, you can compare a number and a string for equality with the == operator. However, you’ll get False as a result:

The integer 2 isn’t equal to the string "2" . Therefore, you get False as a result. You can also use the != operator in the above expression, in which case you’ll get True as a result.

Non-equality comparisons between operands of different data types raise a TypeError exception:

In this example, Python raises a TypeError exception because a less than comparison ( < ) doesn’t make sense between an integer and a string. So, the operation isn’t allowed.

It’s important to note that in the context of comparisons, integer and floating-point values are compatible, and you can compare them.

You’ll typically use and find comparison operators in Boolean contexts like conditional statements and while loops . They allow you to make decisions and define a program’s control flow .

The comparison operators work on several types of operands, such as numbers, strings, tuples, and lists. In the following sections, you’ll explore the differences.

Probably, the more straightforward comparisons in Python and in math are those involving integer numbers. They allow you to count real objects, which is a familiar day-to-day task. In fact, the non-negative integers are also called natural numbers . So, comparing this type of number is probably pretty intuitive, and doing so in Python is no exception.

Consider the following examples that compare integer numbers:

In the first set of examples, you define two variables, a and b , to run a few comparisons between them. The value of a is less than the value of b . So, every comparison expression returns the expected Boolean value. The second set of examples uses two values that are equal, and again, you get the expected results.

Comparing floating-point numbers is a bit more complicated than comparing integers. The value stored in a float object may not be precisely what you’d think it would be. For that reason, it’s bad practice to compare floating-point values for exact equality using the == operator.

Consider the example below:

Yikes! The internal representation of this addition isn’t exactly equal to 3.3 , as you can see in the final example. So, comparing x to 3.3 with the equality operator returns False .

To compare floating-point numbers for equality, you need to use a different approach. The preferred way to determine whether two floating-point values are equal is to determine whether they’re close to one another, given some tolerance.

The math module from the standard library provides a function conveniently called isclose() that will help you with float comparison. The function takes two numbers and tests them for approximate equality:

In this example, you use the isclose() function to compare x and 3.3 for approximate equality. This time, you get True as a result because both numbers are close enough to be considered equal.

For further details on using isclose() , check out the Find the Closeness of Numbers With Python isclose() section in The Python math Module: Everything You Need to Know .

You can also use the comparison operators to compare Python strings in your code. In this context, you need to be aware of how Python internally compares string objects. In practice, Python compares strings character by character using each character’s Unicode code point . Unicode is Python’s default character set .

You can use the built-in ord() function to learn the Unicode code point of any character in Python. Consider the following examples:

The uppercase "A" has a lower Unicode point than the lowercase "a" . So, "A" is less than "a" . In the end, Python compares characters using integer numbers. So, the same rules that Python uses to compare integers apply to string comparison.

When it comes to strings with several characters, Python runs the comparison character by character in a loop.

The comparison uses lexicographical ordering , which means that Python compares the first item from each string. If their Unicode code points are different, this difference determines the comparison result. If the Unicode code points are equal, then Python compares the next two characters, and so on, until either string is exhausted:

In this example, Python compares both operands character by character. When it reaches the end of the string, it compares "o" and "O" . Because the lowercase letter has a greater Unicode code point, the first version of the string is greater than the second.

You can also compare strings of different lengths:

In this example, Python runs a character-by-character comparison as usual. If it runs out of characters, then the shorter string is less than the longer one. This also means that the empty string is the smallest possible string.

In your Python journey, you can also face the need to compare lists with other lists and tuples with other tuples. These data types also support the standard comparison operators. Like with strings, when you use a comparison operator to compare two lists or two tuples, Python runs an item-by-item comparison.

Note that Python applies specific rules depending on the type of the contained items. Here are some examples that compare lists and tuples of integer values:

In these examples, you compare lists and tuples of numbers using the standard comparison operators. When comparing these data types, Python runs an item-by-item comparison.

For example, in the first expression above, Python compares the 2 in the left operand and the 2 in the right operand. Because they’re equal, Python continues comparing 3 and 3 to conclude that both lists are equal. The same thing happens in the second example, where you compare tuples containing the same data.

It’s important to note that you can actually compare lists to tuples using the == and != operators. However, you can’t compare lists and tuples using the < , > , <= , and >= operators:

Python supports equality comparison between lists and tuples. However, it doesn’t support the rest of the comparison operators, as you can conclude from the final two examples. If you try to use them, then you get a TypeError telling you that the operation isn’t supported.

You can also compare lists and tuples of different lengths:

In the first two examples, you get True as a result because 5 is less than 8 . That fact is sufficient for Python to solve the comparison. In the second pair of examples, you get False . This result makes sense because the compared sequences don’t have the same length, so they can’t be equal.

In the final pair of examples, Python compares 5 with 5 . They’re equal, so the comparison continues. Because there are no more values to compare in the right-hand operands, Python concludes that the left-hand operands are greater.

As you can see, comparing lists and tuples can be tricky. It’s also an expensive operation that, in the worst case, requires traversing two entire sequences. Things get more complex and expensive when the contained items are also sequences. In those situations, Python will also have to compare items in a value-by-value manner, which adds cost to the operation.

Boolean Operators and Expressions in Python

Python has three Boolean or logical operators: and , or , and not . They define a set of operations denoted by the generic operators AND , OR , and NOT . With these operators, you can create compound conditions.

In the following sections, you’ll learn how the Python Boolean operators work. Especially, you’ll learn that some of them behave differently when you use them with Boolean values or with regular objects as operands.

You’ll find many objects and expressions that are of Boolean type or bool , as Python calls this type. In other words, many objects evaluate to True or False , which are the Python Boolean values.

For example, when you evaluate an expression using a comparison operator, the result of that expression is always of bool type:

In this example, the expression age > 18 returns a Boolean value, which you store in the is_adult variable. Now is_adult is of bool type, as you can see after calling the built-in type() function.

You can also find Python built-in and custom functions that return a Boolean value. This type of function is known as a predicate function. The built-in all() , any() , callable() , and isinstance() functions are all good examples of this practice.

Consider the following examples:

In this code snippet, you first define a variable called number using your old friend the assignment operator. Then you create another variable called validation_conditions . This variable holds a tuple of expressions. The first expression uses isinstance() to check whether number is an integer value.

The second is a compound expression that combines the modulo ( % ) and equality ( == ) operators to create a condition that checks whether the input value is an even number. In this condition, the modulo operator returns the remainder of dividing number by 2 , and the equality operator compares the result with 0 , returning True or False as the comparison’s result.

Then you use the all() function to determine if all the conditions are true. In this example, because number = 42 , the conditions are true, and all() returns True . You can play with the value of number if you’d like to experiment a bit.

In the final two examples, you use the callable() function. As its name suggests, this function allows you to determine whether an object is callable . Being callable means that you can call the object with a pair of parentheses and appropriate arguments, as you’d call any Python function.

The number variable isn’t callable, and the function returns False , accordingly. In contrast, the print() function is callable, so callable() returns True .

All the previous discussion is the basis for understanding how the Python logical operators work with Boolean operands.

Logical expressions involving and , or , and not are straightforward when the operands are Boolean. Here’s a summary. Note that x and y represent Boolean operands:

This table summarizes the truth value of expressions that you can create using the logical operators with Boolean operands. There’s something to note in this summary. Unlike and and or , which are binary operators, the not operator is unary, meaning that it operates on one operand. This operand must always be on the right side.

Now it’s time to take a look at how the operators work in practice. Here are a few examples of using the and operator with Boolean operands:

In the first example, both operands return True . Therefore, the and expression returns True as a result. In the second example, the left-hand operand is True , but the right-hand operand is False . Because of this, the and operator returns False .

In the third example, the left-hand operand is False . In this case, the and operator immediately returns False and never evaluates the 3 == 3 condition. This behavior is called short-circuit evaluation . You’ll learn more about it in a moment.

Note: Short-circuit evaluation is also called McCarthy evaluation in honor of computer scientist John McCarthy .

In the final example, both conditions return False . Again, and returns False as a result. However, because of the short-circuit evaluation, the right-hand expression isn’t evaluated.

What about the or operator? Here are a few examples that demonstrate how it works:

In the first three examples, at least one of the conditions returns True . In all cases, the or operator returns True . Note that if the left-hand operand is True , then or applies short-circuit evaluation and doesn’t evaluate the right-hand operand. This makes sense. If the left-hand operand is True , then or already knows the final result. Why would it need to continue the evaluation if the result won’t change?

In the final example, both operands are False , and this is the only situation where or returns False . It’s important to note that if the left-hand operand is False , then or has to evaluate the right-hand operand to arrive at a final conclusion.

Finally, you have the not operator, which negates the current truth value of an object or expression:

If you place not before an expression, then you get the inverse truth value. When the expression returns True , you get False . When the expression evaluates to False , you get True .

There is a fundamental behavior distinction between not and the other two Boolean operators. In a not expression, you always get a Boolean value as a result. That’s not always the rule that governs and and or expressions, as you’ll learn in the Boolean Expressions Involving Other Types of Operands section.

In practice, most Python objects and expressions aren’t Boolean. In other words, most objects and expressions don’t have a True or False value but a different type of value. However, you can use any Python object in a Boolean context, such as a conditional statement or a while loop.

In Python, all objects have a specific truth value. So, you can use the logical operators with all types of operands.

Python has well-established rules to determine the truth value of an object when you use that object in a Boolean context or as an operand in an expression built with logical operators. Here’s what the documentation says about this topic:

By default, an object is considered true unless its class defines either a __bool__() method that returns False or a __len__() method that returns zero, when called with the object. Here are most of the built-in objects considered false: constants defined to be false: None and False . zero of any numeric type: 0 , 0.0 , 0j , Decimal(0) , Fraction(0, 1) empty sequences and collections: '' , () , [] , {} , set() , range(0) ( Source )

You can determine the truth value of an object by calling the built-in bool() function with that object as an argument. If bool() returns True , then the object is truthy . If bool() returns False , then it’s falsy .

For numeric values, you have that a zero value is falsy, while a non-zero value is truthy:

Python considers the zero value of all numeric types falsy. All the other values are truthy, regardless of how close to zero they are.

Note: Instead of a function, bool() is a class. However, because Python developers typically use this class as a function, you’ll find that most people refer to it as a function rather than as a class. Additionally, the documentation lists this class on the built-in functions page. This is one of those cases where practicality beats purity .

When it comes to evaluating strings, you have that an empty string is always falsy, while a non-empty string is truthy:

Note that strings containing white spaces are also truthy in Python’s eyes. So, don’t confuse empty strings with whitespace strings.

Finally, built-in container data types, such as lists, tuples, sets , and dictionaries , are falsy when they’re empty. Otherwise, Python considers them truthy objects:

To determine the truth value of container data types, Python relies on the .__len__() special method . This method provides support for the built-in len() function, which you can use to determine the number of items in a given container.

In general, if .__len__() returns 0 , then Python considers the container a falsy object, which is consistent with the general rules you’ve just learned before.

All the discussion about the truth value of Python objects in this section is key to understanding how the logical operators behave when they take arbitrary objects as operands.

You can also use any objects, such as numbers or strings, as operands to and , or , and not . You can even use combinations of a Boolean object and a regular one. In these situations, the result depends on the truth value of the operands.

Note: Boolean expressions that combine two Boolean operands are a special case of a more general rule that allows you to use the logical operators with all kinds of operands. In every case, you’ll get one of the operands as a result.

You’ve already learned how Python determines the truth value of objects. So, you’re ready to dive into creating expressions with logic operators and regular objects.

To start off, below is a table that summarizes what you get when you use two objects, x and y , in an and expression:

It’s important to emphasize a subtle detail in the above table. When you use and in an expression, you don’t always get True or False as a result. Instead, you get one of the operands. You only get True or False if the returned operand has either of these values.

Here are some code examples that use integer values. Remember that in Python, the zero value of numeric types is falsy. The rest of the values are truthy:

In the first expression, the left-hand operand ( 3 ) is truthy. So, you get the right-hand operand ( 4 ) as a result.

In the second example, the left-hand operand ( 0 ) is falsy, and you get it as a result. In this case, Python applies the short-circuit evaluation technique. It already knows that the whole expression is false because 0 is falsy, so Python returns 0 immediately without evaluating the right-hand operand.

In the final expression, the left-hand operand ( 3 ) is truthy. Therefore Python needs to evaluate the right-hand operand to make a conclusion. As a result, you get the right-hand operand, no matter what its truth value is.

Note: To dive deeper into the and operator, check out Using the “and” Boolean Operator in Python .

When it comes to using the or operator, you also get one of the operands as a result. This is what happens for two arbitrary objects, x and y :

Again, the expression x or y doesn’t evaluate to either True or False . Instead, it returns one of its operands, x or y .

As you can conclude from the above table, if the left-hand operand is truthy, then you get it as a result. Otherwise, you get the second operand. Here are some examples that demonstrate this behavior:

In the first example, the left-hand operand is truthy, and or immediately returns it. In this case, Python doesn’t evaluate the second operand because it already knows the final result. In the second example, the left-hand operand is falsy, and Python has to evaluate the right-hand one to determine the result.

In the last example, the left-hand operand is truthy, and that fact defines the result of the expression. There’s no need to evaluate the right-hand operand.

An expression like x or y is truthy if either x or y is truthy, and falsy if both x and y are falsy. This type of expression returns the first truthy operand that it finds. If both operands are falsy, then the expression returns the right-hand operand. To see this latter behavior in action, consider the following example:

In this specific expression, both operands are falsy. So, the or operator returns the right-hand operand, and the whole expression is falsy as a result.

Note: To learn more about the or operator, check out Using the “or” Boolean Operator in Python .

Finally, you have the not operator. You can also use this one with any object as an operand. Here’s what happens:

The not operator has a uniform behavior. It always returns a Boolean value. This behavior differs from its sibling operators, and and or .

Here are some code examples:

In the first example, the operand, 3 , is truthy from Python’s point of view. So, the operator returns False . In the second example, the operand is falsy, and not returns True .

Note: To better understand the not operator, check out Using the “not” Boolean Operator in Python .

In summary, the Python not operator negates the truth value of an object and always returns a Boolean value. This latter behavior differs from the behavior of its sibling operators and and or , which return operands rather than Boolean values.

So far, you’ve seen expressions with only a single or or and operator and two operands. However, you can also create compound logical expressions with multiple logical operators and operands.

To illustrate how to create a compound expression using or , consider the following toy example:

This expression returns the first truthy value. If all the preceding x variables are falsy, then the expression returns the last value, xn .

Note: In an expression like the one above, Python uses short-circuit evaluation . The operands are evaluated in order from left to right. As soon as one is found to be true, the entire expression is known to be true. At that point, Python stops evaluating operands. The value of the entire expression is that of the x that terminates the evaluation.

To help demonstrate short-circuit evaluation, suppose that you have an identity function , f() , that behaves as follows:

• Takes a single argument
• Displays the function and its argument on the screen
• Returns the argument as its return value

Here’s the code to define this function and also a few examples of how it works:

The f() function displays its argument, which visually confirms whether you called the function. It also returns the argument as you passed it in the call. Because of this behavior, you can make the expression f(arg) be truthy or falsy by specifying a value for arg that’s truthy or falsy, respectively.

Now, consider the following compound logical expression:

In this example, Python first evaluates f(0) , which returns 0 . This value is falsy. The expression isn’t true yet, so the evaluation continues from left to right. The next operand, f(False) , returns False . That value is also falsy, so the evaluation continues.

Next up is f(1) . That evaluates to 1 , which is truthy. At that point, Python stops the evaluation because it already knows that the entire expression is truthy. Consequently, Python returns 1 as the value of the expression and never evaluates the remaining operands, f(2) and f(3) . You can confirm from the output that the f(2) and f(3) calls don’t occur.

A similar behavior appears in an expression with multiple and operators like the following one:

This expression is truthy if all the operands are truthy. If at least one operand is falsy, then the expression is also falsy.

In this example, short-circuit evaluation dictates that Python stops evaluating as soon as an operand happens to be falsy. At that point, the entire expression is known to be false. Once that’s the case, Python stops evaluating operands and returns the falsy operand that terminated the evaluation.

Here are two examples that confirm the short-circuiting behavior:

In both examples, the evaluation stops at the first falsy term— f(False) in the first case, f(0.0) in the second case—and neither the f(2) nor the f(3) call occurs. In the end, the expressions return False and 0.0 , respectively.

If all the operands are truthy, then Python evaluates them all and returns the last (rightmost) one as the value of the expression:

In the first example, all the operands are truthy. The expression is also truthy and returns the last operand. In the second example, all the operands are truthy except for the last one. The expression is falsy and returns the last operand.

As you dig into Python, you’ll find that there are some common idiomatic patterns that exploit short-circuit evaluation for conciseness of expression, performance, and safety. For example, you can take advantage of this type of evaluation for:

• Avoiding an exception
• Providing a default value
• Skipping a costly operation

To illustrate the first point, suppose you have two variables, a and b , and you want to know whether the division of b by a results in a number greater than 0 . In this case, you can run the following expression or condition:

This code works. However, you need to account for the possibility that a might be 0 , in which case you’ll get an exception :

In this example, the divisor is 0 , which makes Python raise a ZeroDivisionError exception. This exception breaks your code. You can skip this error with an expression like the following:

When a is 0 , a != 0 is false. Python’s short-circuit evaluation ensures that the evaluation stops at that point, which means that (b / a) never runs, and the error never occurs.

Using this technique, you can implement a function to determine whether an integer is divisible by another integer:

In this function, if b is 0 , then a / b isn’t defined. So, the numbers aren’t divisible. If b is different from 0 , then the result will depend on the remainder of the division.

Selecting a default value when a specified value is falsy is another idiom that takes advantage of the short-circuit evaluation feature of Python’s logical operators.

For example, say that you have a variable that’s supposed to contain a country’s name. At some point, this variable can end up holding an empty string. If that’s the case, then you’d like the variable to hold a default county name. You can also do this with the or operator:

If country is non-empty, then it’s truthy. In this scenario, the expression will return the first truthy value, which is country in the first or expression. The evaluation stops, and you get "Canada" as a result.

On the other hand, if country is an empty string , then it’s falsy. The evaluation continues to the next operand, default_country , which is truthy. Finally, you get the default country as a result.

Another interesting use case for short-circuit evaluation is to avoid costly operations while creating compound logical expressions. For example, if you have a costly operation that should only run if a given condition is false, then you can use or like in the following snippet:

In this construct, your clean_data() function represents a costly operation. Because of short-circuit evaluation, this function will only run when data_is_clean is false, which means that your data isn’t clean.

Another variation of this technique is when you want to run a costly operation if a given condition is true. In this case, you can use the and operator:

In this example, the and operator evaluates data_is_updated . If this variable is true, then the evaluation continues, and the process_data() function runs. Otherwise, the evaluation stops, and process_data() never runs.

Sometimes you have a compound expression that uses the and operator to join comparison expressions. For example, say that you want to determine if a number is in a given interval. You can solve this problem with a compound expression like the following:

In this example, you use the and operator to join two comparison expressions that allow you to find out if number is in the interval from 0 to 10 , both included.

In Python, you can make this compound expression more concise by chaining the comparison operators together. For example, the following chained expression is equivalent to the previous compound one:

This expression is more concise and readable than the original expression. You can quickly realize that this code is checking if the number is between 0 and 10 . Note that in most programming languages, this chained expression doesn’t make sense. In Python, it works like a charm.

In other programming languages, this expression would probably start by evaluating 0 <= number , which is true. This true value would then be compared with 10 , which doesn’t make much sense, so the expression fails.

Python internally processes this type of expression as an equivalent and expression, such as 0 <= number and number <= 10 . That’s why you get the correct result in the example above.

Python has what it calls conditional expressions . These kinds of expressions are inspired by the ternary operator that looks like a ? b : c and is used in other programming languages. This construct evaluates to b if the value of a is true, and otherwise evaluates to c . Because of this, sometimes the equivalent Python syntax is also known as the ternary operator.

However, in Python, the expression looks more readable:

This expression returns expression_1 if the condition is true and expression_2 otherwise. Note that this expression is equivalent to a regular conditional like the following:

So, why does Python need this syntax? PEP 308 introduced conditional expressions as an effort to avoid the prevalence of error-prone attempts to achieve the same effect of a traditional ternary operator using the and and or operators in an expression like the following:

However, this expression doesn’t work as expected, returning expression_2 when expression_1 is falsy.

Some Python developers would avoid the syntax of conditional expressions in favor of a regular conditional statement. In any case, this syntax can be handy in some situations because it provides a concise tool for writing two-way conditionals.

Here’s an example of how to use the conditional expression syntax in your code:

When day is equal to "Sunday" , the condition is true and you get the first expression, "11AM" , as a result. If the condition is false, then you get the second expression, "9AM" . Note that similarly to the and and or operators, the conditional expression returns the value of one of its expressions rather than a Boolean value.

Python provides two operators, is and is not , that allow you to determine whether two operands have the same identity . In other words, they let you check if the operands refer to the same object. Note that identity isn’t the same thing as equality. The latter aims to check whether two operands contain the same data.

Note: To learn more about the difference between identity and equality, check out Python ‘!=’ Is Not ‘is not’: Comparing Objects in Python .

Here’s a summary of Python’s identity operators. Note that x and y are variables that point to objects:

These two Python operators are keywords instead of odd symbols. This is part of Python’s goal of favoring readability in its syntax.

Here’s an example of two variables, x and y , that refer to objects that are equal but not identical:

In this example, x and y refer to objects whose value is 1001 . So, they’re equal. However, they don’t reference the same object. That’s why the is operator returns False . You can check an object’s identity using the built-in id() function:

As you can conclude from the id() output, x and y don’t have the same identity. So, they’re different objects, and because of that, the expression x is y returns False . In other words, you get False because you have two different instances of 1001 stored in your computer’s memory.

When you make an assignment like y = x , Python creates a second reference to the same object. Again, you can confirm that with the id() function or the is operator:

In this example, a and b hold references to the same object, the string "Hello, Pythonista!" . Therefore, the id() function returns the same identity when you call it with a and b . Similarly, the is operator returns True .

Note: You should note that, on your computer, you’ll get a different identity number when you call id() in the example above. The key detail is that the identity number will be the same for a and b .

Finally, the is not operator is the opposite of is . So, you can use is not to determine if two names don’t refer to the same object:

In the first example, because x and y point to different objects in your computer’s memory, the is not operator returns True . In the second example, because a and b are references to the same object, the is not operator returns False .

Note: The syntax not x is y also works the same as x is not y . However, the former syntax looks odd and is difficult to read. That’s why Python recognizes is not as an operator and encourages its use for readability.

Again, the is not operator highlights Python’s readability goals. In general, both identity operators allow you to write checks that read as plain English.

Sometimes you need to determine whether a value is present in a container data type, such as a list, tuple, or set. In other words, you may need to check if a given value is or is not a member of a collection of values. Python calls this kind of check a membership test .

Note: For a deep dive into how Python’s membership tests work, check out Python’s “in” and “not in” Operators: Check for Membership .

Membership tests are quite common and useful in programming. As with many other common operations, Python has dedicated operators for membership tests. The table below lists the membership operators in Python:

As usual, Python favors readability by using English words as operators instead of potentially confusing symbols or combinations of symbols.

Note: The syntax not value in collection also works in Python. However, this syntax looks odd and is difficult to read. So, to keep your code clean and readable, you should use value not in collection , which almost reads as plain English.

The Python in and not in operators are binary. This means that you can create membership expressions by connecting two operands with either operator. However, the operands in a membership expression have particular characteristics:

• Left operand : The value that you want to look for in a collection of values
• Right operand : The collection of values where the target value may be found

To better understand the in operator, below you have two demonstrative examples consisting of determining whether a value is in a list:

The first expression returns True because 5 is in the list of numbers. The second expression returns False because 8 isn’t in the list.

The not in membership operator runs the opposite test as the in operator. It allows you to check whether an integer value is not in a collection of values:

In the first example, you get False because 5 is in the target list. In the second example, you get True because 8 isn’t in the list of values. This may sound like a tongue twister because of the negative logic. To avoid confusion, remember that you’re trying to determine if the value is not part of a given collection of values.

There are two operators in Python that acquire a slightly different meaning when you use them with sequence data types, such as lists, tuples, and strings. With these types of operands, the + operator defines a concatenation operator , and the * operator represents the repetition operator :

Both operators are binary. The concatenation operator takes two sequences as operands and returns a new sequence of the same type. The repetition operator takes a sequence and an integer number as operands. Like in regular multiplication, the order of the operands doesn’t alter the repetition’s result.

Note: To learn more about concatenating string objects, check out Efficient String Concatenation in Python .

Here are some examples of how the concatenation operator works in practice:

In the first example, you use the concatenation operator ( + ) to join two strings together. The operator returns a completely new string object that combines the two original strings.

In the second example, you concatenate two tuples of letters together. Again, the operator returns a new tuple object containing all the items from the original operands. In the final example, you do something similar but this time with two lists.

Note: To learn more about concatenating lists, check out the Concatenating Lists section in the tutorial Python’s list Data Type: A Deep Dive With Examples .

When it comes to the repetition operator, the idea is to repeat the content of a given sequence a certain number of times. Here are a few examples:

In the first example, you use the repetition operator ( * ) to repeat the "Hello" string three times. In the second example, you change the order of the operands by placing the integer number on the left and the target string on the right. This example shows that the order of the operands doesn’t affect the result.

The next examples use the repetition operators with a tuple and a list, respectively. In both cases, you get a new object of the same type containing the items in the original sequence repeated three times.

Regular assignment statements with the = operator don’t have a return value, as you already learned. Instead, the assignment operator creates or updates variables. Because of this, the operator can’t be part of an expression.

Since Python 3.8 , you have access to a new operator that allows for a new type of assignment. This new assignment is called assignment expression or named expression . The new operator is called the walrus operator , and it’s the combination of a colon and an equal sign ( := ).

Note: The name walrus comes from the fact that this operator resembles the eyes and tusks of a walrus lying on its side. For a deep dive into how this operator works, check out The Walrus Operator: Python’s Assignment Expressions .

Unlike regular assignments, assignment expressions do have a return value, which is why they’re expressions . So, the operator accomplishes two tasks:

• Returns the expression’s result
• Assigns the result to a variable

The walrus operator is also a binary operator. Its left-hand operand must be a variable name, and its right-hand operand can be any Python expression. The operator will evaluate the expression, assign its value to the target variable, and return the value.

The general syntax of an assignment expression is as follows:

This expression looks like a regular assignment. However, instead of using the assignment operator ( = ), it uses the walrus operator ( := ). For the expression to work correctly, the enclosing parentheses are required in most use cases. However, in certain situations, you won’t need them. Either way, they won’t hurt you, so it’s safe to use them.

Assignment expressions come in handy when you want to reuse the result of an expression or part of an expression without using a dedicated assignment to grab this value beforehand. It’s particularly useful in the context of a conditional statement. To illustrate, the example below shows a toy function that checks the length of a string object:

In this example, you use a conditional statement to check whether the input string has fewer than 8 characters.

The assignment expression, (n := len(string)) , computes the string length and assigns it to n . Then it returns the value that results from calling len() , which finally gets compared with 8 . This way, you guarantee that you have a reference to the string length to use in further operations.

Bitwise operators treat operands as sequences of binary digits and operate on them bit by bit. Currently, Python supports the following bitwise operators:

As you can see in this table, most bitwise operators are binary, which means that they expect two operands. The bitwise NOT operator ( ~ ) is the only unary operator because it expects a single operand, which should always appear at the right side of the expression.

You can use Python’s bitwise operators to manipulate your data at its most granular level, the bits. These operators are commonly useful when you want to write low-level algorithms, such as compression, encryption, and others.

Note: For a deep dive into the bitwise operators, check out Bitwise Operators in Python . You can also check out Build a Maze Solver in Python Using Graphs for an example of using bitwise operators to construct a binary file format.

Here are some examples that illustrate how some of the bitwise operators work in practice:

In the first example, you use the bitwise AND operator. The commented lines begin with # and provide a visual representation of what happens at the bit level. Note how each bit in the result is the logical AND of the bits in the corresponding position of the operands.

The second example shows how the bitwise OR operator works. In this case, the resulting bits are the logical OR test of the corresponding bits in the operands.

In all the examples, you’ve used the built-in bin() function to display the result as a binary object. If you don’t wrap the expression in a call to bin() , then you’ll get the integer representation of the output.

Up to this point, you’ve coded sample expressions that mostly use one or two different types of operators. However, what if you need to create compound expressions that use several different types of operators, such as comparison, arithmetic, Boolean, and others? How does Python decide which operation runs first?

Consider the following math expression:

There might be ambiguity in this expression. Should Python perform the addition 20 + 4 first and then multiply the result by 10 ? Should Python run the multiplication 4 * 10 first, and the addition second?

Because the result is 60 , you can conclude that Python has chosen the latter approach. If it had chosen the former, then the result would be 240 . This follows a standard algebraic rule that you’ll find in virtually all programming languages.

All operators that Python supports have a precedence compared to other operators. This precedence defines the order in which Python runs the operators in a compound expression.

In an expression, Python runs the operators of highest precedence first. After obtaining those results, Python runs the operators of the next highest precedence. This process continues until the expression is fully evaluated. Any operators of equal precedence are performed in left-to-right order.

Here’s the order of precedence of the Python operators that you’ve seen so far, from highest to lowest:

Operators at the top of the table have the highest precedence, and those at the bottom of the table have the lowest precedence. Any operators in the same row of the table have equal precedence.

Getting back to your initial example, Python runs the multiplication because the multiplication operator has a higher precedence than the addition one.

Here’s another illustrative example:

In the example above, Python first raises 3 to the power of 4 , which equals 81 . Then, it carries out the multiplications in order from left to right: 2 * 81 = 162 and 162 * 5 = 810 .

You can override the default operator precedence using parentheses to group terms as you do in math. The subexpressions in parentheses will run before expressions that aren’t in parentheses.

Here are some examples that show how a pair of parentheses can affect the result of an expression:

In the first example, Python computes the expression 20 + 4 first because it’s wrapped in parentheses. Then Python multiplies the result by 10 , and the expression returns 240 . This result is completely different from what you got at the beginning of this section.

In the second example, Python evaluates 4 * 5 first. Then it raises 3 to the power of the resulting value. Finally, Python multiplies the result by 2 , returning 6973568802 .

There’s nothing wrong with making liberal use of parentheses, even when they aren’t necessary to change the order of evaluation. Sometimes it’s a good practice to use parentheses because they can improve your code’s readability and relieve the reader from having to recall operator precedence from memory.

Consider the following example:

Here the parentheses are unnecessary, as the comparison operators have higher precedence than and . However, some might find the parenthesized version clearer than the version without parentheses:

On the other hand, some developers might prefer this latter version of the expression. It’s a matter of personal preference. The point is that you can always use parentheses if you feel that they make your code more readable, even if they aren’t necessary to change the order of evaluation.

So far, you’ve learned that a single equal sign ( = ) represents the assignment operator and allows you to assign a value to a variable. Having a right-hand operand that contains other variables is perfectly valid, as you’ve also learned. In particular, the expression to the right of the assignment operator can include the same variable that’s on the left of the operand.

That last sentence may sound confusing, so here’s an example that clarifies the point:

In this example, total is an accumulator variable that you use to accumulate successive values. You should read this example as total is equal to the current value of total plus 5 . This expression effectively increases the value of total , which is now 15 .

Note that this type of assignment only makes sense if the variable in question already has a value. If you try the assignment with an undefined variable, then you get an error:

In this example, the count variable isn’t defined before the assignment, so it doesn’t have a current value. In consequence, Python raises a NameError exception to let you know about the issue.

This type of assignment helps you create accumulators and counter variables, for example. Therefore, it’s quite a common task in programming. As in many similar cases, Python offers a more convenient solution. It supports a shorthand syntax called augmented assignment :

In the highlighted line, you use the augmented addition operator ( += ). With this operator, you create an assignment that’s fully equivalent to total = total + 5 .

Python supports many augmented assignment operators. In general, the syntax for this type of assignment looks something like this:

Note that the dollar sign ( $ ) isn’t a valid Python operator. In this example, it’s a placeholder for a generic operator. The above statement works as follows:

• Evaluate expression to produce a value.
• Run the operation defined by the operator that prefixes the assignment operator ( = ), using the current value of variable and the return value of expression as operands.
• Assign the resulting value back to variable .

The table below shows a summary of the augmented operators for arithmetic operations:

As you can conclude from this table, all the arithmetic operators have an augmented version in Python. You can use these augmented operators as a shortcut when creating accumulators, counters, and similar objects.

Did the augmented arithmetic operators look neat and useful to you? The good news is that there are more. You also have augmented bitwise operators in Python:

Finally, the concatenation and repetition operators have augmented variations too. These variations behave differently with mutable and immutable data types:

Note that the augmented concatenation operator works on two sequences, while the augmented repetition operator works on a sequence and an integer number.

Now you know what operators Python supports and how to use them. Operators are symbols, combinations of symbols, or keywords that you can use along with Python objects to build different types of expressions and perform computations in your code.

In this tutorial, you’ve learned:

• What Python’s arithmetic operators are and how to use them in arithmetic expressions
• What Python’s comparison , Boolean , identity , membership operators are
• How to write expressions using comparison, Boolean, identity, and membership operators
• Which bitwise operators Python supports and how to use them
• How to combine and repeat sequences using the concatenation and repetition operators
• What the augmented assignment operators are and how they work

In other words, you’ve covered an awful lot of ground! If you’d like a handy cheat sheet that can jog your memory on all that you’ve learned, then click the link below:

With all this knowledge about operators, you’re better prepared as a Python developer. You’ll be able to write better and more robust expressions in your code.

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Operators and Expressions in Python (Cheat Sheet)

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Statement vs Expression – What's the Difference in Programming?

Learning the syntax of a programming language is key if you want to use that language effectively. This is true for both new and experienced developers.

And one of the most important things to pay attention to while learning a programming language is whether the code you're dealing with is a statement or an expression.

It can sometimes be confusing to differentiate between statements and expressions in programming. So this article is meant to simplify the differences so that you can improve your programming skills and become a better developer.

What is an Expression in Programming?

An expression is any word or group of words or symbols that is a value. In programming, an expression is a value, or anything that executes and ends up being a value.

It is necessary to understand that a value is unique. For example, const , let , 2 , 4 , s , a , true , false , and world are values because each of them is unique in meaning or character.

Let's look at some code as an example:

Judging from the code above, const , price , = , and 500 are expressions because each of them has a definite and unique meaning or value. But if we take all of them together const price = 500 - then we have a statement.

Let's look at another example:

Looking at the code above, you can see an anonymous function is assigned to a variable. Oh, wait! You might know that any function is a statement. Can it also be an expression?

Yes! A "function" and a "class" are both statements and expressions because they can perform actions (do or not do tasks) and still execute to a value.

This brings us to statements – so what are they?

What is a Statement in Programming?

A statement is a group of expressions and/or statements that you design to carry out a task or an action.

Statements are two-sided – that is, they either do tasks or don't do them. Any statement that can return a value is automatically qualified to be used as an expression. That is why a function or class is a statement and also an expression in JavaScript.

If you look at the example of the function under the section on expressions, you can see it is assigned and execute to a value passed to a variable. That is why it is an expression in that case.

Examples of Statements in Programming

Inline statements.

The whole of the code above is a statement because it carries out the task of assigning $2000 to amount . It is safe to say a line of code is a statement because most compilers or interpreters don't execute any standalone expression.

Block statements

Look at the below if statement:

The if statement is a statement because it helps us check whether I love you or not. As I have said before, it is two-sided: this code finds out whether "I love you" or not, and that is why it is a statement. Also, it doesn't return any value but it can create side effects.

Here's a loop statement:

In short, any loop is a statement because if it can only do the tasks it is meant to do or not – does loop and doesn't loop. But a loop can't execute to a value in the end. They can only have side effects in JavaScript. Once they can execute to a value in a programming language, then they can also be used as an expression.

For example, you can use forloop and if statement as expressions in Python.

There is also an "IF" expression in Python. That means that something that is a statement in one language can be an expression (or both statement and expression) in another.

Look at the below function statement:

We declare the function add(firstNumber, secondNumber) and it returns a value. The function is called with two arguments as in add(2, 3) by declaration and so it is a statement. If you pay close attention, you will realize that calling the function as a statement is useless since it has no side effect.

Hey, stop! How can we turn it into an expression? Oh yeah, we can do it like this:

Though the function is now an expression the way it is called above, the whole of the code is still a statement.

Check out this class statement:

You can see that we declare the class "Person" and instantiate and assign it to "User" immediately. So, it is used as an expression.

Now, let's use it as a statement:

A class is similar to a function in the sense that it can be declared, assigned, or used as an operand just like a class. So, a class is a statement and/or an expression.

The Main Differences Between an Expression and a Statement in Programming

Expressions can be assigned or used as operands, while statements can only be declared.

Statements create side effects to be useful, while expressions are values or execute to values.

Expressions are unique in meaning, while statements are two-sided in execution. For example, 1 has a certain value while go( ) may be executed or not.

Statements are the whole structure, while expressions are the building blocks. For example, a line or a block of code is a statement.

Why You Should Know the Difference

First of all, understanding the difference between statements and expressions should make learning new programming languages less surprising. If you're used to JavaScript, you may be surprised by Python's ability to assign an if statement as a variable which is not possible in JavaScript.

Second, it makes it easy to use programming paradigms across different programming languages.

For example, a JavaScript "if statement" cannot be used as an expression because it can't execute to a value – it can only create side effects. Yet, you can use the ternary operator if you want to avoid the side effects of using an if statement in JavaScript.

For this reason, you can understand why some programmers avoid if statements by using the ternary operator in JavaScript. It is because they want to avoid side effects .

It also makes your realize why you have to be always careful about the scope of your variables whenever you use a statement. This is true because statements mostly have side effects to be useful, and it is reasonable to understand the scope of your variables and operations. For example,

Hey wait! What would be logged in the console if you ran the code above?

Tell yourself the answer first and then paste the code in the console to confirm. If you you're wrong, you need to learn more about scope and side effects. But if you're right, try to make those functions a bit better to avoid the confusion they may generate.

Knowing the difference also helps you to easily identify non-composable and composable syntaxes (functions, classes, modules, and so on) of a programming language. This makes porting your experience from one programming language to another more interesting and direct.

Wrapping Up

Now that you understand the difference between expressions and statements in programming, and you know why understanding the differences is important, you can identify pieces of code as expressions or statements while coding.

Next time, we'll go even further and help make learning a second programming language easier.

Go and get things done now! See you soon.

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Assignment operators (C# reference)

• 11 contributors

The assignment operator = assigns the value of its right-hand operand to a variable, a property , or an indexer element given by its left-hand operand. The result of an assignment expression is the value assigned to the left-hand operand. The type of the right-hand operand must be the same as the type of the left-hand operand or implicitly convertible to it.

The assignment operator = is right-associative, that is, an expression of the form

is evaluated as

The following example demonstrates the usage of the assignment operator with a local variable, a property, and an indexer element as its left-hand operand:

The left-hand operand of an assignment receives the value of the right-hand operand. When the operands are of value types , assignment copies the contents of the right-hand operand. When the operands are of reference types , assignment copies the reference to the object.

This is called value assignment : the value is assigned.

ref assignment

Ref assignment = ref makes its left-hand operand an alias to the right-hand operand, as the following example demonstrates:

In the preceding example, the local reference variable arrayElement is initialized as an alias to the first array element. Then, it's ref reassigned to refer to the last array element. As it's an alias, when you update its value with an ordinary assignment operator = , the corresponding array element is also updated.

The left-hand operand of ref assignment can be a local reference variable , a ref field , and a ref , out , or in method parameter. Both operands must be of the same type.

Compound assignment

For a binary operator op , a compound assignment expression of the form

is equivalent to

except that x is only evaluated once.

Compound assignment is supported by arithmetic , Boolean logical , and bitwise logical and shift operators.

Null-coalescing assignment

You can use the null-coalescing assignment operator ??= to assign the value of its right-hand operand to its left-hand operand only if the left-hand operand evaluates to null . For more information, see the ?? and ??= operators article.

Operator overloadability

A user-defined type can't overload the assignment operator. However, a user-defined type can define an implicit conversion to another type. That way, the value of a user-defined type can be assigned to a variable, a property, or an indexer element of another type. For more information, see User-defined conversion operators .

A user-defined type can't explicitly overload a compound assignment operator. However, if a user-defined type overloads a binary operator op , the op= operator, if it exists, is also implicitly overloaded.

C# language specification

For more information, see the Assignment operators section of the C# language specification .

• C# operators and expressions
• ref keyword
• Use compound assignment (style rules IDE0054 and IDE0074)

Additional resources

• C Data Types
• C Operators
• C Input and Output
• C Control Flow
• C Functions
• C Preprocessors
• C File Handling
• C Cheatsheet
• C Interview Questions

Assignment Operators in C

Assignment operators are used for assigning value to a variable. The left side operand of the assignment operator is a variable and right side operand of the assignment operator is a value. The value on the right side must be of the same data-type of the variable on the left side otherwise the compiler will raise an error.

Different types of assignment operators are shown below:

1. “=”: This is the simplest assignment operator. This operator is used to assign the value on the right to the variable on the left. Example:

2. “+=” : This operator is combination of ‘+’ and ‘=’ operators. This operator first adds the current value of the variable on left to the value on the right and then assigns the result to the variable on the left. Example:

If initially value stored in a is 5. Then (a += 6) = 11.

3. “-=” This operator is combination of ‘-‘ and ‘=’ operators. This operator first subtracts the value on the right from the current value of the variable on left and then assigns the result to the variable on the left. Example:

If initially value stored in a is 8. Then (a -= 6) = 2.

4. “*=” This operator is combination of ‘*’ and ‘=’ operators. This operator first multiplies the current value of the variable on left to the value on the right and then assigns the result to the variable on the left. Example:

If initially value stored in a is 5. Then (a *= 6) = 30.

5. “/=” This operator is combination of ‘/’ and ‘=’ operators. This operator first divides the current value of the variable on left by the value on the right and then assigns the result to the variable on the left. Example:

If initially value stored in a is 6. Then (a /= 2) = 3.

Below example illustrates the various Assignment Operators:

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C Programming: error: assignment to expression with array type

Hello I am a beginner in C and I was working with structures when i have this problem:

With this one I have this error ( error: assignment to expression with array type ). So I decided to do it using pointers and it actually working this is the code I used:

So the question is what is the difference between both of them since strings are pointers. thank you ..

• This has been probably answered before in SO... –  mcleod_ideafix Commented May 15, 2020 at 20:28
• Does this answer your question? C - Difference between "char var[]" and "char *var"? –  mcleod_ideafix Commented May 15, 2020 at 20:30
• 1 "since strings are pointers". That's not a correct assumption. –  mcleod_ideafix Commented May 15, 2020 at 20:31

3 Answers 3

Strings are not pointers, they are sequences of characters terminated with a null byte. These sequences are stored in arrays, which are not pointers either, but objects that live in memory and to which a pointer can point. A pointer is a variable that contains the address of an object in memory.

In the first example, you try and store a string into an array with = . C does not support this type of assignment and complains with an explicit error message. You can copy the string into the array with strcpy , assuming the destination array is large enough to store the bytes of the string, including the null terminator. "reda" uses 5 bytes, so it is OK to copy it into E[0].nom :

The second example uses a different approach: nom is now defined as a pointer to char . Defining the array E in main leaves it uninitialized, so you can only read from the pointers after you set their value to point to actual arrays of char somewhere in memory. E[0].nom = "reda"; does just that: set the address of the string literal "reda" into the member nom of E[0] . The string literal will be placed by the compiler into an array of its own, including a null terminator, and that must not be changed by the program.

• Thank you a lot . i just have another question: if we use scanf("%s",E[0].nom) it does work properly even when we are not using nom as a pointer but just as an array of chars –  Reda El Idrissi Commented May 15, 2020 at 21:07
• @RedaElIdrissi: to avoid buffer overruns, you should write scanf("%19s", E[0].nom) –  chqrlie Commented May 15, 2020 at 21:34
• @RedaElIdrissi: it works because when you pass an array to a function, the function receives a pointer to the first element of the array. –  chqrlie Commented May 15, 2020 at 21:39

First part, you try to copy two array of character (string is not a pointer, it is array of character that is terminated by null character \0 ).

If you want to copy value of an array to another, you can use memcpy , but for string, you can also use strcpy .

Second part, you make the pointer point to string literal. The pointer points to first character of the string ( r in this case).

You can see How to copy a string using a pointer

And Assigning strings to pointer in C Language

It is a lower-level concept actually.

If we say char nom[20] than its memory location will be decided at compile time and it will use stack memory (see the difference between stack memory and heap memory to gain more grasp on lover level programming). so we need to use it with indexes.

On the other hand if we create a string using the double quotes method (or the second way in which you used pointers). The double quoted strings are identified as const char* by the compiler and the string is placed at the heap memory rather than the stack memory. moreover their memory location is not decided at compile time.

a very efficient way of testing the difference between these two types of strings is that when you use sizeof() with char[20] nom it will return 20 * sizeof(char) which evaluates to 20 without any doubt. it clearly states that whether you use only 5 characters or 2 characters it will always occupy space of 20 charaters (in stack memory)

but in case of const char* if you use sizeof operator on this variable than it will return the size of the pointer variable which depends on the hardware (32bit will have 4bytes pointer) and (64bit will have 8bytes pointer). So, the sizeof operator do not shows how many characters are stored in the pointer. it just shows the size of pointer. but the benefit of using const char* is that only take memory that is required if 4 characters are stored than it will consume 5 byte memory (last byte is additionally added in case of strings) and in case of 8 characters it will consume 9 bytes and so on but note that the characters will be stored in heap(memory)

What you have been doing in the first problem is that you were assigning a string that is only stored on heap to a data types that is only stored on stack. it is not the matter of string. it the matter of memory model

I hope you might have understood. I not i can elaborate more in comments.

an additional information. Stack Memory is faster compared to the Heap Memory.

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