The importance of real code

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:

match = re.match(data)
group = match.group(1) if match else None

they would write:

group = re.match(data).group(1) if re.match(data) else None

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

match1 = pattern1.match(data)
match2 = pattern2.match(data)
if match1:
    result = match1.group(1)
elif match2:
    result = match2.group(2)
else:
    result = None

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:

match1 = pattern1.match(data)
if match1:
    result = match1.group(1)
else:
    match2 = pattern2.match(data)
    if match2:
        result = match2.group(2)
    else:
        result = None

Exceptional cases

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

• Unparenthesized assignment expressions are prohibited at the top level of an expression statement. Example:

  y := f(x)  # INVALID
  (y := f(x))  # Valid, though not recommended
  

  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.

• Unparenthesized assignment expressions are prohibited at the top level of the right hand side of an assignment statement. Example:

  y0 = y1 := f(x)  # INVALID
  y0 = (y1 := f(x))  # Valid, though discouraged
  

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

• Unparenthesized assignment expressions are prohibited for the value of a keyword argument in a call. Example:

  foo(x = y := f(x))  # INVALID
  foo(x=(y := f(x)))  # Valid, though probably confusing
  

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

• Unparenthesized assignment expressions are prohibited at the top level of a function default value. Example:

  def foo(answer = p := 42):  # INVALID
      ...
  def foo(answer=(p := 42)):  # Valid, though not great style
      ...
  

  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).

• Unparenthesized assignment expressions are prohibited as annotations for arguments, return values and assignments. Example:

  def foo(answer: p := 42 = 5):  # INVALID
      ...
  def foo(answer: (p := 42) = 5):  # Valid, but probably never useful
      ...
  

  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.

• Unparenthesized assignment expressions are prohibited in lambda functions. Example:

  (lambda: x := 1) # INVALID
  lambda: (x := 1) # Valid, but unlikely to be useful
  (x := lambda: 1) # Valid
  lambda line: (m := re.match(pattern, line)) and m.group(1) # Valid
  

  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.

• Assignment expressions inside of f-strings require parentheses. Example:

  >>> f'{(x:=10)}'  # Valid, uses assignment expression
  '10'
  >>> x = 10
  >>> f'{x:=10}'    # Valid, passes '=10' to formatter
  '        10'
  

  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.

Scope of the target

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:

if any((comment := line).startswith('#') for line in lines):
    print("First comment:", comment)
else:
    print("There are no comments")

if all((nonblank := line).strip() == '' for line in lines):
    print("All lines are blank")
else:
    print("First non-blank line:", nonblank)

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

# Compute partial sums in a list comprehension
total = 0
partial_sums = [total := total + v for v in values]
print("Total:", total)

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:

[[(j := j) for i in range(5)] for j in range(5)] # INVALID
[i := 0 for i, j in stuff]                       # INVALID
[i+1 for i in (i := stuff)]                      # INVALID

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:

[False and (i := 0) for i, j in stuff]     # INVALID
[i for i, j in stuff if True or (j := 1)]  # INVALID

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):

[i+1 for i in (j := stuff)]                    # INVALID
[i+1 for i in range(2) for j in (k := stuff)]  # INVALID
[i+1 for i in [j for j in (k := stuff)]]       # INVALID
[i+1 for i in (lambda: (j := stuff))()]        # INVALID

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:

class Example:
    [(j := i) for i in range(5)]  # INVALID

(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.

Relative precedence of :=

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:

# INVALID
x := 0

# Valid alternative
(x := 0)

# INVALID
x = y := 0

# Valid alternative
x = (y := 0)

# Valid
len(lines := f.readlines())

# Valid
foo(x := 3, cat='vector')

# INVALID
foo(cat=category := 'vector')

# Valid alternative
foo(cat=(category := 'vector'))

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:

# Valid
if any(len(longline := line) >= 100 for line in lines):
    print("Extremely long line:", longline)

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.)

Change to evaluation order

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.)

Differences between assignment expressions and assignment statements

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)
  

Examples from the Python standard library

Simplifying list comprehensions

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

results = [(x, y, x/y) for x in input_data if (y := f(x)) > 0]

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

stuff = [[y := f(x), x/y] for x in range(5)]

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

Capturing condition values

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

# Loop-and-a-half
while (command := input("> ")) != "quit":
    print("You entered:", command)

# Capturing regular expression match objects
# See, for instance, Lib/pydoc.py, which uses a multiline spelling
# of this effect
if match := re.search(pat, text):
    print("Found:", match.group(0))
# The same syntax chains nicely into 'elif' statements, unlike the
# equivalent using assignment statements.
elif match := re.search(otherpat, text):
    print("Alternate found:", match.group(0))
elif match := re.search(third, text):
    print("Fallback found:", match.group(0))

# Reading socket data until an empty string is returned
while data := sock.recv(8192):
    print("Received data:", data)

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.

Fork

An example from the low-level UNIX world:

if pid := os.fork():
    # Parent code
else:
    # Child code

Changing the scope rules for comprehensions

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.

Alternative spellings

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

1. EXPR as NAME:

   stuff = [[f(x) as y, x/y] for x in range(5)]
   

   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.
   • In all other situations where an as clause is allowed, even readers with intermediary skills are led to anticipate that clause (however optional) by the keyword that starts the line, and the grammar ties that keyword closely to the as clause:
     • 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.

   • The parallel cadence between
     • NAME = EXPR
     • if NAME := EXPR

     reinforces the visual recognition of assignment expressions.

2. EXPR -> NAME:

   stuff = [[f(x) -> y, x/y] for x in range(5)]
   

   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.

3. Adorning statement-local names with a leading dot:

   stuff = [[(f(x) as .y), x/.y] for x in range(5)] # with "as"
   stuff = [[(.y := f(x)), x/.y] for x in range(5)] # with ":="
   

   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.

4. Adding a where: to any statement to create local name bindings:

   value = x**2 + 2*x where:
       x = spam(1, 4, 7, q)
   

   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:).

5. TARGET from EXPR:

   stuff = [[y from f(x), x/y] for x in range(5)]
   

   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.

Special-casing conditional statements

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:

if re.search(pat, text) as match:
    print("Found:", match.group(0))

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.

Special-casing comprehensions

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

1. where, let, or given:

   stuff = [(y, x/y) where y = f(x) for x in range(5)]
   stuff = [(y, x/y) let y = f(x) for x in range(5)]
   stuff = [(y, x/y) given y = f(x) for x in range(5)]
   

   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).

2. with NAME = EXPR:

   stuff = [(y, x/y) with y = f(x) for x in range(5)]
   

   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.

3. with EXPR as NAME:

   stuff = [(y, x/y) with f(x) as y for x in range(5)]
   

   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.

Lowering operator precedence

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:

pos = -1
while pos := buffer.find(search_term, pos + 1) >= 0:
    ...

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).

Allowing commas to the right

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

(point := (x, y))
(point := x, y)

(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:

foo(x := 1, y)
foo(x := (1, y))

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

Always requiring parentheses

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:

# Top level in if
if match := pattern.match(line):
    return match.group(1)

# Short call
len(lines := f.readlines())

Why not just turn existing assignment into an expression?

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.

With assignment expressions, why bother with assignment statements?

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.

Why not use a sublocal scope and prevent namespace pollution?

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])

A numeric example

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):

while a > (d := x // a**(n-1)):
    a = ((n-1)*a + d) // n
return a

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:

while True:
    d = x // a**(n-1)
    if a <= d:
        break
    a = ((n-1)*a + d) // n
return a