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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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terryjreedy Nov 15, 2020
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JelleZijlstra Apr 5, 2022
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Merge branch 'main' into walrus
JelleZijlstra Oct 3, 2022
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Update Doc/reference/expressions.rst
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@JelleZijlstra
JelleZijlstra authored Oct 3, 2022
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6 changes: 3 additions & 3 deletions Doc/reference/expressions.rst
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Expand Up @@ -1767,9 +1767,9 @@ Or, when processing a file stream in chunks:
process(chunk)

Assignment expressions must be surrounded by parentheses when used
as sub-expressions in subscript, slicing, conditional, lambda,
keyword-argument, generator, and comprehension-if expressions. They
can only be used as is in if and while statements and in decorators.
as sub-expressions in slicing, conditional, lambda,
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@lysnikolaou lysnikolaou Oct 3, 2022

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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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@JelleZijlstra JelleZijlstra Oct 3, 2022

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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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@lysnikolaou lysnikolaou Oct 3, 2022

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Correct! I mixed up indexes/slices. Sorry!
keyword-argument, and comprehension-if expressions. They
can be used unparenthesized in all other places, including ``if`` and ``while`` statements.

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