Mailman 3 PEP 640: Unused variable syntax. - Python-Dev

20 Oct 2020

      One of the problems I have with the Pattern Matching proposal (PEP 622
originally, now PEPs 634, 635, 636) is the special-casing of '_' to not
actually assign to the name, which is a subtle but meaningful divergence
from the rest of Python. In discussions with the authors I proposed using
'?' instead *and* extending that to existing unpacking syntax. The PEP
authors were understandably a little hesitant to tack that onto their
already quite extensive proposal, so I suggested making it a
separate proposal. That proposal is at
https://www.python.org/dev/peps/pep-0640 (unless that hasn't updated yet,
in which case it's at
https://github.com/python/peps/blob/master/pep-0640.rst), and also included
below.

This proposal doesn't necessarily require pattern matching to be accepted
-- the new syntax stands well enough on its own -- but I'm recommending
this *not* be accepted if pattern matching using the same syntax is not
also accepted. The benefit without pattern matching is real but small, and
in my opinion it's not worth the added complexity.

The PEP:

PEP: 640
Title: Unused variable syntax
Author: Thomas Wouters 
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 04-Oct-2020
Python-Version: 3.10
Post-History:

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. 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.

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.

Reference Implementation
========================

A prototype implementation exists at
https://github.com/Yhg1s/cpython/tree/nonassign.

Rejected Ideas
==============

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?

Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.

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PEP 640: Unused variable syntax.

Thomas Wouters

Steven D'Aprano

Thomas Wouters

Steven D'Aprano

Steve Dower

Chris Jerdonek

Thomas Wouters

Paul Moore

Thomas Wouters

Paul Moore

Thomas Wouters

Paul Moore

Steve Dower

Paul Sokolovsky

Larry Hastings

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participants (7)