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gh-86482: Document assignment expression need for ()s #23291

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Merged
merged 8 commits into from
Oct 6, 2022
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gh-86482: Document assignment expression need for ()s #23291

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bpo-42316: Document assignment expression need for ()s
terryjreedy Nov 15, 2020
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JelleZijlstra Apr 5, 2022
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Update Doc/reference/expressions.rst
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6 changes: 6 additions & 0 deletions 6 Doc/reference/expressions.rst
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Expand Up @@ -1766,6 +1766,12 @@ Or, when processing a file stream in chunks:
while chunk := file.read(9000):
process(chunk)

Assignment expressions must be surrounded by parentheses when used
as sub-expressions in slicing, conditional, lambda,
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Unparenthesized assignment expressions in slices were implemented in #23317, so they are allowed since 3.10. Also, assignment expression need to parenthesized in assert and with statements.

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Unparenthesized walruses are allowed in index expressions but not slices. This is current 3.12:

>>> x[y:=3]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined
>>> x[y:=3:z:=4]
  File "<stdin>", line 1
    x[y:=3:z:=4]
          ^
SyntaxError: invalid syntax

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Correct! I mixed up indexes/slices. Sorry!

keyword-argument, and comprehension-if expressions.
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In all other places where they can be used, parentheses are not required,
including in ``if`` and ``while`` statements.

.. versionadded:: 3.8
See :pep:`572` for more details about assignment expressions.

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1 change: 1 addition & 0 deletions 1 Misc/NEWS.d/next/Core and Builtins/2020-11-15-02-08-43.bpo-42316.LqdkWK.rst
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Document some places where an assignment expression needs parentheses.
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