Apologies for letting this languish; life has an annoying habit of getting in the way now and then.
Feedback from the previous rounds has been incorporated. From here, the most important concern and question is: Is there any other syntax or related proposal that ought to be mentioned here? If this proposal is rejected, it should be rejected with a full set of alternatives.
Text of PEP is below; formatted version will be live shortly (if it isn't already) at:
PEP: 572 Title: Syntax for Statement-Local Name Bindings Author: Chris Angelico firstname.lastname@example.org 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
Programming is all about reusing code rather than duplicating it. When an expression needs to be used twice in quick succession but never again, it is convenient to assign it to a temporary name with small scope. By permitting name bindings to exist within a single statement only, we make this both convenient and safe against name collisions.
When a subexpression is used multiple times in a list comprehension, there are currently several ways to spell this, none of which is universally accepted as ideal. A statement-local name allows any subexpression to be temporarily captured and then used multiple times.
Additionally, this syntax can in places be used to remove the need to write an infinite loop with a ``break`` in it. Capturing part of a ``while`` loop's condition can improve the clarity of the loop header while still making the actual value available within the loop body.
Syntax and semantics ====================
In any context where arbitrary Python expressions can be used, a **named expression** can appear. This must be parenthesized for clarity, and is of the form ``(expr as NAME)`` where ``expr`` is any valid Python expression, and ``NAME`` is a simple name.
The value of such a named expression is the same as the incorporated expression, with the additional side-effect that NAME is bound to that value for the remainder of the current statement.
Just as function-local names shadow global names for the scope of the function, statement-local names shadow other names for that statement. (They can technically also shadow each other, though actually doing this should not be encouraged.)
Assignment to statement-local names is ONLY through this syntax. Regular assignment to the same name will remove the statement-local name and affect the name in the surrounding scope (function, class, or module).
Statement-local names never appear in locals() or globals(), and cannot be closed over by nested functions.
Execution order and its consequences ------------------------------------
Since the statement-local name binding lasts from its point of execution to the end of the current statement, this can potentially cause confusion when the actual order of execution does not match the programmer's expectations. Some examples::
# A simple statement ends at the newline or semicolon. a = (1 as y) print(y) # NameError
# The assignment ignores the SLNB - this adds one to 'a' a = (a + 1 as a)
# Compound statements usually enclose everything... if (re.match(...) as m): print(m.groups(0)) print(m) # NameError
# ... except when function bodies are involved... if (input("> ") as cmd): def run_cmd(): print("Running command", cmd) # NameError
# ... but function *headers* are executed immediately if (input("> ") as cmd): def run_cmd(cmd=cmd): # Capture the value in the default arg print("Running command", cmd) # Works
Function bodies, in this respect, behave the same way they do in class scope; assigned names are not closed over by method definitions. Defining a function inside a loop already has potentially-confusing consequences, and SLNBs do not materially worsen the existing situation.
Differences from regular assignment statements ----------------------------------------------
Using ``(EXPR as NAME)`` is similar to ``NAME = EXPR``, but has a number of important distinctions.
* Assignment is a statement; an SLNB is an expression whose value is the same as the object bound to the new name. * SLNBs disappear at the end of their enclosing statement, at which point the name again refers to whatever it previously would have. SLNBs can thus shadow other names without conflict (although deliberately doing so will often be a sign of bad code). * SLNBs cannot be closed over by nested functions, and are completely ignored for this purpose. * SLNBs do not appear in ``locals()`` or ``globals()``. * An SLNB cannot be the target of any form of assignment, including augmented. Attempting to do so will remove the SLNB and assign to the fully-scoped name.
In many respects, an SLNB is akin to a local variable in an imaginary nested function, except that the overhead of creating and calling a function is bypassed. As with names bound by ``for`` loops inside list comprehensions, SLNBs cannot "leak" into their surrounding scope.
Example usage =============
These list comprehensions are all approximately equivalent::
# Calling the function twice stuff = [[f(x), x/f(x)] for x in range(5)]
# External helper function def pair(x, value): return [value, x/value] stuff = [pair(x, f(x)) for x in range(5)]
# 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)]]
# Iterating over a genexp stuff = [[y, x/y] for x, y in ((x, f(x)) for x in range(5))]
# Expanding the comprehension into a loop stuff =  for x in range(5): y = f(x) stuff.append([y, x/y])
# Wrapping the loop in a generator function def g(): for x in range(5): y = f(x) yield [y, x/y] stuff = list(g())
# Using a statement-local name stuff = [[(f(x) as y), x/y] for x in range(5)]
If calling ``f(x)`` is expensive or has side effects, the clean operation of the list comprehension gets muddled. Using a short-duration name binding retains the simplicity; while the extra ``for`` loop does achieve this, it does so at the cost of dividing the expression visually, putting the named part at the end of the comprehension instead of the beginning.
Statement-local name bindings can be used in any context, but should be avoided where regular assignment can be used, just as ``lambda`` should be avoided when ``def`` is an option. As the name's scope extends to the full current statement, even a block statement, this can be used to good effect in the header of an ``if`` or ``while`` statement::
# Current Python, not caring about function return value while input("> ") != "quit": print("You entered a command.")
# Current Python, capturing return value - four-line loop header while True: command = input("> "); if command == "quit": break print("You entered:", command)
# Proposed alternative to the above while (input("> ") as command) != "quit": print("You entered:", command)
# See, for instance, Lib/pydoc.py if (re.search(pat, text) as match): print("Found:", match.group(0))
while (sock.read() as data): 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.
Performance costs =================
The cost of SLNBs must be kept to a minimum, particularly when they are not used; the normal case MUST NOT be measurably penalized. SLNBs are expected to be uncommon, and using many of them in a single function should definitely be discouraged. Thus the current implementation uses a linked list of SLNB cells, with the absence of such a list being the normal case. This list is used for code compilation only; once a function's bytecode has been baked in, execution of that bytecode has no performance cost compared to regular assignment.
Other Python implementations may choose to do things differently, but a zero run-time cost is strongly recommended, as is a minimal compile-time cost in the case where no SLNBs are used.
Forbidden special cases =======================
In two situations, the use of SLNBs makes no sense, and could be confusing due to the ``as`` keyword already having a different meaning in the same context.
1. Exception catching::
try: ... except (Exception as e1) as e2: ...
The expression ``(Exception as e1)`` has the value ``Exception``, and creates an SLNB ``e1 = Exception``. This is generally useless, and creates the potential confusion in that these two statements do quite different things:
except (Exception as e1): except Exception as e2:
The latter captures the exception **instance**, while the former captures the ``Exception`` **type** (not the type of the raised exception).
2. Context managers::
lock = threading.Lock() with (lock as l) as m: ...
This captures the original Lock object as ``l``, and the result of calling its ``__enter__`` method as ``m``. As with ``except`` statements, this creates a situation in which parenthesizing an expression subtly changes its semantics, with the additional pitfall that this will frequently work (when ``x.__enter__()`` returns x, eg with file objects).
Both of these are forbidden; creating SLNBs in the headers of these statements will result in a SyntaxError.
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.
1. ``where``, ``let``, ``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.
4. ``EXPR as NAME`` without parentheses::
stuff = [[f(x) as y, x/y] for x in range(5)]
Omitting the parentheses from this PEP's proposed syntax introduces many syntactic ambiguities. Requiring them in all contexts leaves open the option to make them optional in specific situations where the syntax is unambiguous (cf generator expressions as sole parameters in function calls), but there is no plausible way to make them optional everywhere.
5. Adorning statement-local names with a leading dot::
stuff = [[(f(x) as .y), x/.y] for x in range(5)]
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. This syntax is quite viable, and could be promoted to become the current recommendation if its advantages are found to outweigh its cost.
6. Allowing ``(EXPR as NAME)`` to assign to any form of name.
This is exactly the same as the promoted proposal, save that the name is bound in the same scope that it would otherwise have. Any expression can assign to any name, just as it would if the ``=`` operator had been used. Such variables would leak out of the statement into the enclosing function, subject to the regular behaviour of comprehensions (since they implicitly create a nested function, the name binding would be restricted to the comprehension itself, just as with the names bound by ``for`` loops).
7. Enhancing ``if`` and ``while`` syntax to permit the capture of their conditions::
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.
Discrepancies in the current implementation ===========================================
1. SLNBs are implemented using a special (and mostly-invisible) name mangling. They may sometimes appear in globals() and/or locals() with their simple or mangled names (but buggily and unreliably). They should be suppressed as though they were guinea pigs.
2. The forbidden special cases do not yet raise SyntaxError.
..  Proof of concept / reference implementation (https://github.com/Rosuav/cpython/tree/statement-local-variables)
This document has been placed in the public domain.
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