The most notable change since last posting is that the assignment target is no longer as flexible as with the statement form of assignment, but is restricted to a simple name. Note that the reference implementation has not been updated. ChrisA PEP: 572 Title: Assignment Expressions Author: Chris Angelico Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 28-Feb-2018 Python-Version: 3.8 Post-History: 28-Feb-2018, 02-Mar-2018, 23-Mar-2018, 04-Apr-2018, 17-Apr-2018, 25-Apr-2018 Abstract ======== This is a proposal for creating a way to assign to variables within an expression. Additionally, the precise scope of comprehensions is adjusted, to maintain consistency and follow expectations. Rationale ========= 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. Merely introducing a way to assign as an expression would create bizarre edge cases around comprehensions, though, and to avoid the worst of the confusions, we change the definition of comprehensions, causing some edge cases to be interpreted differently, but maintaining the existing behaviour in the majority of situations. Syntax and semantics ==================== In any context 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, 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:: # Handle a matched regex if (match := pattern.search(data)) is not None: ... # A more explicit alternative to the 2-arg form of iter() invocation while (value := read_next_item()) is not None: ... # Share a subexpression between a comprehension filter clause and its output filtered_data = [y for x in data if (y := f(x)) is not None] Differences from regular assignment statements ---------------------------------------------- Most importantly, since ``:=`` is an expression, it can be used in contexts where statements are illegal, including lambda functions and comprehensions. An assignment statement can assign to multiple targets, left-to-right:: x = y = z = 0 The equivalent assignment expression is parsed as separate binary operators, and is therefore processed right-to-left, as if it were spelled thus:: assert 0 == (x := (y := (z := 0))) Statement assignment can include annotations. This would be syntactically noisy in expressions, and is of minor importance. An annotation can be given separately from the assignment if needed:: x:str = "" # works (x:str := "") # SyntaxError x:str # possibly before a loop (x := "") # fine Augmented assignment is not supported in expression form:: >>> x +:= 1 File "<stdin>", line 1 x +:= 1 ^ SyntaxError: invalid syntax Statement assignment is able to set attributes and subscripts, but expression assignment is restricted to names. (This restriction may be relaxed in a future version of Python.) Otherwise, the semantics of assignment are identical in statement and expression forms. Alterations to comprehensions ----------------------------- The current behaviour of list/set/dict comprehensions and generator expressions has some edge cases that would behave strangely if an assignment expression were to be used. Therefore the proposed semantics are changed, removing the current edge cases, and instead altering their behaviour *only* in a class scope. As of Python 3.7, the outermost iterable of any comprehension is evaluated in the surrounding context, and then passed as an argument to the implicit function that evaluates the comprehension. Under this proposal, the entire body of the comprehension is evaluated in its implicit function. Names not assigned to within the comprehension are located in the surrounding scopes, as with normal lookups. As one special case, a comprehension at class scope will **eagerly bind** any name which is already defined in the class scope. A list comprehension can be unrolled into an equivalent function. With Python 3.7 semantics:: numbers = [x + y for x in range(3) for y in range(4)] # Is approximately equivalent to def <listcomp>(iterator): result = [] for x in iterator: for y in range(4): result.append(x + y) return result numbers = <listcomp>(iter(range(3))) Under the new semantics, this would instead be equivalent to:: def <listcomp>(): result = [] for x in range(3): for y in range(4): result.append(x + y) return result numbers = <listcomp>() When a class scope is involved, a naive transformation into a function would prevent name lookups (as the function would behave like a method):: class X: names = ["Fred", "Barney", "Joe"] prefix = "> " prefixed_names = [prefix + name for name in names] With Python 3.7 semantics, this will evaluate the outermost iterable at class scope, which will succeed; but it will evaluate everything else in a function:: class X: names = ["Fred", "Barney", "Joe"] prefix = "> " def <listcomp>(iterator): result = [] for name in iterator: result.append(prefix + name) return result prefixed_names = <listcomp>(iter(names)) The name ``prefix`` is thus searched for at global scope, ignoring the class name. Under the proposed semantics, this name will be eagerly bound; and the same early binding then handles the outermost iterable as well. The list comprehension is thus approximately equivalent to:: class X: names = ["Fred", "Barney", "Joe"] prefix = "> " def <listcomp>(names=names, prefix=prefix): result = [] for name in names: result.append(prefix + name) return result prefixed_names = <listcomp>() With list comprehensions, this is unlikely to cause any confusion. With generator expressions, this has the potential to affect behaviour, as the eager binding means that the name could be rebound between the creation of the genexp and the first call to ``next()``. It is, however, more closely aligned to normal expectations. The effect is ONLY seen with names that are looked up from class scope; global names (eg ``range()``) will still be late-bound as usual. One consequence of this change is that certain bugs in genexps will not be detected until the first call to ``next()``, where today they would be caught upon creation of the generator. Recommended use-cases ===================== 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)] # There are a number of less obvious ways to spell this in current # versions of Python, such as: # Inline helper function stuff = [(lambda y: [y,x/y])(f(x)) for x in range(5)] # Extra 'for' loop - potentially could be optimized internally stuff = [[y, x/y] for x in range(5) for y in [f(x)]] # Using a mutable cache object (various forms possible) c = {} stuff = [[c.update(y=f(x)) or c['y'], x/c['y']] for x in range(5)] In all cases, the name is local to the comprehension; like iteration variables, it cannot leak out into the surrounding context. Capturing condition values -------------------------- Assignment expressions can be used to good effect in the header of an ``if`` or ``while`` statement:: # Proposed syntax 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)) # Reading socket data until an empty string is returned while data := sock.read(): print("Received data:", data) # Equivalent in current Python, not caring about function return value while input("> ") != "quit": print("You entered a command.") # To capture the return value in current Python demands a four-line # loop header. while True: command = input("> "); if command == "quit": break print("You entered:", command) 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. 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. 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 ``except`` and ``with`` statements (with different semantics), this would create unnecessary confusion or require special-casing (eg to forbid assignment within the headers of these statements). 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`` and ``except`` statements, but otherwise is equivalent. 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 (eg 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 (eg 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 ``=``. Migration path ============== The semantic changes to list/set/dict comprehensions, and more so to generator expressions, may potentially require migration of code. In many cases, the changes simply make legal what used to raise an exception, but there are some edge cases that were previously legal and now are not, and a few corner cases with altered semantics. The Outermost Iterable ---------------------- As of Python 3.7, the outermost iterable in a comprehension is special: it is evaluated in the surrounding context, instead of inside the comprehension. Thus it is permitted to contain a ``yield`` expression, to use a name also used elsewhere, and to reference names from class scope. Also, in a genexp, the outermost iterable is pre-evaluated, but the rest of the code is not touched until the genexp is first iterated over. Class scope is now handled more generally (see above), but if other changes require the old behaviour, the iterable must be explicitly elevated from the comprehension:: # Python 3.7 def f(x): return [x for x in x if x] def g(): return [x for x in [(yield 1)]] # With PEP 572 def f(x): return [y for y in x if y] def g(): sent_item = (yield 1) return [x for x in [sent_item]] This more clearly shows that it is g(), not the comprehension, which is able to yield values (and is thus a generator function). The entire comprehension is consistently in a single scope. The following expressions would, in Python 3.7, raise exceptions immediately. With the removal of the outermost iterable's special casing, they are now equivalent to the most obvious longhand form:: gen = (x for x in rage(10)) # NameError gen = (x for x in 10) # TypeError (not iterable) gen = (x for x in range(1/0)) # ZeroDivisionError def <genexp>(): for x in rage(10): yield x gen = <genexp>() # No exception yet tng = next(gen) # NameError Open questions ============== Importing names into comprehensions ----------------------------------- A list comprehension can use and update local names, and they will retain their values from one iteration to another. It would be convenient to use this feature to create rolling or self-effecting data streams:: progressive_sums = [total := total + value for value in data] This will fail with UnboundLocalError due to ``total`` not being initalized. Simply initializing it outside of the comprehension is insufficient - unless the comprehension is in class scope:: class X: total = 0 progressive_sums = [total := total + value for value in data] At other scopes, it may be beneficial to have a way to fetch a value from the surrounding scope. Should this be automatic? Should it be controlled with a keyword? Hypothetically (and using no new keywords), this could be written:: total = 0 progressive_sums = [total := total + value import nonlocal total for value in data] Translated into longhand, this would become:: total = 0 def <listcomp>(total=total): result = [] for value in data: result.append(total := total + value) return result progressive_sums = <listcomp>() ie utilizing the same early-binding technique that is used at class scope. Frequently Raised Objections ============================ 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. This could be used to create ugly code! --------------------------------------- So can anything else. This is a tool, and it is up to the programmer to use it where it makes sense, and not use it where superior constructs can be used. 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. Names bound within a comprehension are local to that comprehension, even in the outermost iterable, and can thus be used freely without polluting the surrounding namespace. (The author wishes to thank Guido van Rossum and Christoph Groth for their suggestions to move the proposal in this direction. [2]_) Style guide recommendations =========================== 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. 1. If either assignment statements or assignment expressions can be used, prefer statements; they are a clear declaration of intent. 2. If using assignment expressions would lead to ambiguity about execution order, restructure it to use statements instead. Acknowledgements ================ The author wishes to thank Guido van Rossum and Nick Coghlan for their considerable contributions to this proposal, and to members of the core-mentorship mailing list for assistance with implementation. References ========== .. [1] Proof of concept / reference implementation (https://github.com/Rosuav/cpython/tree/assignment-expressions) .. [2] Pivotal post regarding inline assignment semantics (https://mail.python.org/pipermail/python-ideas/2018-March/049409.html) Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: