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PEP 640 -- Unused variable syntax

PEP:640
Title:Unused variable syntax
Author:Thomas Wouters <thomas at python.org>
Status:Draft
Type:Standards Track
Created:04-Oct-2020
Python-Version:3.10
Post-History:19-Oct-2020

Abstract

This PEP proposes new syntax for unused variables, providing a pseudo-name that can be assigned to but not otherwise used. The assignment doesn't actually happen, and the value is discarded instead.

Motivation

In Python it is somewhat common to need to do an assignment without actually needing the result. Conventionally, people use either "_" or a name such as "unused" (or with "unused" as a prefix) for this. It's most common in unpacking assignments:

x, unused, z = range(3)
x, *unused, z = range(10)

It's also used in for loops and comprehensions:

for unused in range(10): ...
[ SpamObject() for unused in range(10) ]

The use of "_" in these cases is probably the most common, but it potentially conflicts with the use of "_" in internationalization, where a call like gettext.gettext() is bound to "_" and used to mark strings for translation.

In the proposal to add Pattern Matching to Python (originally PEP 622, now split into PEP 634, PEP 635 and PEP 636), "_" has an additional special meaning. It is a wildcard pattern, used in places where variables could be assigned to, to indicate anything should be matched but not assigned to anything. The choice of "_" there matches the use of "_" in other languages, but the semantic difference with "_" elsewhere in Python is significant.

This PEP proposes to allow a special token, "?", to be used instead of any valid name in assignment. This has most of the benefits of "_" without affecting other uses of that otherwise regular variable. Allowing the use of the same wildcard pattern would make pattern matching and unpacking assignment more consistent with each other.

Rationale

Marking certain variables as unused is a useful tool, as it helps clarity of purpose of the code. It makes it obvious to readers of the code as well as automated linters, that a particular variable is intentionally unused.

However, despite the convention, "_" is not a special variable. The value is still assigned to, the object it refers to is still kept alive until the end of the scope, and it can still be used. Nor is the use of "_" for unused variables entirely ubiquitous, since it conflicts with conventional internationalization, it isn't obvious that it is a regular variable, and it isn't as obviously unused like a variable named "unused".

In the Pattern Matching proposal, the use of "_" for wildcard patterns side-steps the problems of "_" for unused variables by virtue of it being in a separate scope. The only conflict it has with internationalization is one of potential confusion, it will not actually interact with uses of a global variable called "_". However, the special-casing of "_" for this wildcard pattern purpose is still problematic: the different semantics and meaning of "_" inside pattern matching and outside of it means a break in consistency in Python.

Introducing "?" as special syntax for unused variables both inside and outside pattern matching allows us to retain that consistency. It avoids the conflict with internationalization or any other uses of _ as a variable. It makes unpacking assignment align more closely with pattern matching, making it easier to explain pattern matching as an extension of unpacking assignment.

In terms of code readability, using a special token makes it easier to find out what it means ("what does question mark in Python do" versus "why is my _ variable not getting assigned to"), and makes it more obvious that the actual intent is for the value to be unused -- since it is entirely impossible to use it.

Specification

A new token is introduced, "?", or token.QMARK.

The grammar is modified to allow "?" in assignment contexts (star_atom and t_atom in the current grammar), creating a Name AST node with identifier set to NULL.

The AST is modified to allow the Name expression's identifier to be optional (it is currently required). The identifier being empty would only be allowed in a STORE context.

In CPython, the bytecode compiler is modified to emit POP_TOP instead of STORE_NAME for Name nodes with no identifier. Other uses of the Name node are updated to handle the identifier being empty, as appropriate.

The uses of the modified grammar nodes encompass at least the following forms of assignment:

? = ...
x, ?, z = ...
x, *?, z = ...
for ? in range(3): ...  # including comprehension forms
for x, ?, z in matrix: ...  # including comprehension forms
with open(f) as ?: ...
with func() as (x, ?, z): ...

The use of a single "?", not in an unpacking context, is allowed in normal assignment and the with statement. It doesn't really make sense on its own, and it is possible to disallow those specific cases. However, for ? in range(3) clearly has its uses, so for consistency reasons if nothing else it seems more sensible to allow the use of the single "?" in other cases.

Using "?" in augmented assignment (? *= 2) is not allowed, since "?" can only be used for assignment. Having multiple occurrences of "?" is valid, just like when assigning to names, and the assignments do not interfere with each other.

Backwards Compatibility

Introducing a new token means there are no backward compatibility concerns. No valid syntax changes meaning.

"?" is not considered an identifier, so str.isidentifier() does not change.

The AST does change in an incompatible way, as the identifier of a Name token can now be empty. Code using the AST will have to be adjusted accordingly.

How to Teach This

"?" can be introduced along with unpacking assignment, explaining it is special syntax for 'unused' and mentioning that it can also be used in other places. Alternatively, it could be introduced as part of an explanation on assignment in for loops, showing an example where the loop variable is unused.

PEP 636 discusses how to teach "_", and can simply replace "_" with "?", perhaps noting that "?" is similarly usable in other contexts.

Open Issues

Should "?" be allowed in the following contexts:

# imports done for side-effect only.
import os as ?
from os import path as ?

# Function defined for side-effects only (e.g. decorators)
@register_my_func
def ?(...): ...

# Class defined for side-effects only (e.g. decorators, __init_subclass__)
class ?(...): ...

# Parameters defined for unused positional-only arguments:
def f(a, ?, ?): ...
lambda a, ?, ?: ...

# Unused variables with type annotations:
?: int = f()

# Exception handling:
try: ...
except Exception as ?: ...

# With blocks:
with open(f) as ?: ...

Some of these may seem to make sense from a consistency point of view, but practical uses are limited and dubious. Type annotations on "?" and using it with except and with do not seem to make any sense. In the reference implementation, except is not supported (the existing syntax only allows a name) but with is (by virtue of the existing syntax supporting unpacking assignment).

Should this PEP be accepted even if pattern matching is rejected?

Source: https://github.com/python/peps/blob/master/pep-0640.rst