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

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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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7 changes: 7 additions & 0 deletions Doc/reference/expressions.rst
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Expand Up @@ -1766,6 +1766,13 @@ 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
and in ``assert`` and ``with`` statements.
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 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.