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PEP 640: Update post history and fix a bit of markup. (#1664) 

Update post history and fix a bit of markup, and address reviewer comments.
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T. Wouters 2020-10-20 03:58:01 -07:00 committed by GitHub
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@ -6,7 +6,7 @@ Type: Standards Track
Content-Type: text/x-rst Content-Type: text/x-rst
Created: 04-Oct-2020 Created: 04-Oct-2020
Python-Version: 3.10 Python-Version: 3.10
Post-History: Post-History: 19-Oct-2020
Abstract Abstract
======== ========
@ -44,11 +44,11 @@ assigned to anything. The choice of ``"_"`` there matches the use of ``"_"``
in other languages, but the semantic difference with ``"_"`` elsewhere in in other languages, but the semantic difference with ``"_"`` elsewhere in
Python is significant. Python is significant.
This PEP proposes to allow a special token, ``?``, to be used instead. This This PEP proposes to allow a special token, ``"?"``, to be used instead of
has most of the benefits of ``"_"`` without affecting other uses of that any valid name in assignment. This has most of the benefits of ``"_"``
otherwise regular variable. Allowing the use of the same wildcard pattern without affecting other uses of that otherwise regular variable. Allowing
would make pattern matching and unpacking assignment more consistent with the use of the same wildcard pattern would make pattern matching and
each other. unpacking assignment more consistent with each other.
Rationale Rationale
========= =========
@ -74,7 +74,7 @@ special-casing of ``"_"`` for this wildcard pattern purpose is still
problematic: the different semantics *and meaning* of ``"_"`` inside pattern problematic: the different semantics *and meaning* of ``"_"`` inside pattern
matching and outside of it means a break in consistency in Python. matching and outside of it means a break in consistency in Python.
Introducing ``?`` as special syntax for unused variables *both inside and Introducing ``"?"`` as special syntax for unused variables *both inside and
outside pattern matching* allows us to retain that consistency. It avoids outside pattern matching* allows us to retain that consistency. It avoids
the conflict with internationalization *or any other uses of _ as a the conflict with internationalization *or any other uses of _ as a
variable*. It makes unpacking assignment align more closely with pattern variable*. It makes unpacking assignment align more closely with pattern
@ -124,7 +124,9 @@ nothing else it seems more sensible to allow the use of the single ``"?"``
in other cases. in other cases.
Using ``"?"`` in augmented assignment (``? *= 2``) is not allowed, since Using ``"?"`` in augmented assignment (``? *= 2``) is not allowed, since
``"?"`` can only be used for assignment. ``"?"`` 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 Backwards Compatibility
======================= =======================
@ -132,7 +134,8 @@ Backwards Compatibility
Introducing a new token means there are no backward compatibility concerns. Introducing a new token means there are no backward compatibility concerns.
No valid syntax changes meaning. No valid syntax changes meaning.
"?" is not considered an identifier, so str.isidentifier() does not change. ``"?"`` 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 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 token can now be empty. Code using the AST will have to be adjusted
@ -141,14 +144,14 @@ accordingly.
How to Teach This How to Teach This
================= =================
``?`` can be introduced along with unpacking assignment, explaining it is ``"?"`` can be introduced along with unpacking assignment, explaining it is
special syntax for 'unused' and mentioning that it can also be used in other 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 places. Alternatively, it could be introduced as part of an explanation on
assignment in ``for`` loops, showing an example where the loop variable is assignment in ``for`` loops, showing an example where the loop variable is
unused. unused.
PEP 636 discusses how to teach ``"_"``, and can simply replace ``"_"`` with PEP 636 discusses how to teach ``"_"``, and can simply replace ``"_"`` with
``?``, perhaps noting that ``?`` is similarly usable in other contexts. ``"?"``, perhaps noting that ``"?"`` is similarly usable in other contexts.
Reference Implementation Reference Implementation
======================== ========================