|Title:||Syntax for Statement-Local Name Bindings|
|Author:||Chris Angelico <rosuav at gmail.com>|
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.
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 in all retrievals 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 also shadow each other, though actually doing this should be strongly discouraged in style guides.
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.
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
Some of these examples should be considered bad code and rejected by code review and/or linters; they are not, however, illegal.
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.
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 - see also Serhiy's optimization 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.
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.
- What happens if the name has already been used? (x, (1 as x), x) Currently, prior usage functions as if the named expression did not exist (following the usual lookup rules); the new name binding will shadow the other name from the point where it is evaluated until the end of the statement. Is this acceptable? Should it raise a syntax error or warning?
- Syntactic confusion in except statements. While technically unambiguous, it is potentially confusing to humans. In Python 3.7, parenthesizing except (Exception as e): is illegal, and there is no reason to capture the exception type (as opposed to the exception instance, as is done by the regular syntax). Should this be made outright illegal, to prevent confusion? Can it be left to linters? It may also (and independently) be of value to use a subscope for the normal except clause binding, such that except Exception as e: will no longer unbind a previous use of the name e.
- Similar confusion in with statements, with the difference that there is good reason to capture the result of an expression, and it is also very common for __enter__ methods to return self. In many cases, with expr as name: will do the same thing as with (expr as name):, adding to the confusion.
- Should closures be able to refer to statement-local names? Either way, there will be edge cases that make no sense. Assigning to a name will "push through" the SLNB and bind to the regular name; this means that a statement x = x will promote the SLNB to full name, and thus has an impact. Closing over statement-local names, however, introduces scope and lifetime confusions, as it then becomes possible to have two functions in almost the same context, closing over the same name, referring to two different cells.
Proposals of this nature 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.
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).
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.
stuff = [(y, x/y) with f(x) as y for x in range(5)]
As per option 2, but using as in place of the 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.
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.
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.
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.
- 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.
|||Proof of concept / reference implementation (https://github.com/Rosuav/cpython/tree/statement-local-variables)|
This document has been placed in the public domain.