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GeezerVison is showing github.com — gh-89547: Support for nesting special forms like Final by hmc-cs-mdrissi · Pull Request #116096 · python/cpython

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@hmc-cs-mdrissi

@hmc-cs-mdrissi hmc-cs-mdrissi commented Feb 29, 2024 •
edited by bedevere-app Bot
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Resolves this issue. The main goal was to permit ClassVar[Final[int]] and Final[ClassVar[int]]. This drops validation check that Final/ClassVar argument is not a special form.

This does allow some silly cases like ClassVar[ClassVar[int]] but I think it's simpler to allow them then have more complex validation rules at runtime. I did not remove special form check for other forms so Union[ClassVar[int], int] remains forbidden and list[ClassVar[int]] is also forbidden. There already was a test case that list[ClassVar[int]] fails.

I also added couple tests for Annotated to ensure it can nest freely with ClassVar/Final.

  • Issue: Clarify meaning of Final in dataclasses #89547

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@hmc-cs-mdrissi
Add support for nesting special forms like Final
a7ca1f7
@hmc-cs-mdrissi
hmc-cs-mdrissi requested review from AlexWaygood and JelleZijlstra as code owners February 29, 2024 04:11
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ghost commented Feb 29, 2024 •
edited by ghost
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@bedevere-app bedevere-app Bot added the awaiting review label Feb 29, 2024
@bedevere-app bedevere-app Bot mentioned this pull request Feb 29, 2024
Clarify meaning of Final in dataclasses #89547
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@Eclips4 Eclips4 added the topic-typing label Feb 29, 2024
AlexWaygood
AlexWaygood reviewed Feb 29, 2024
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Wow, that's an easy change to make! Not sure if we want to treat this as a new feature or a backportable bugfix — thoughts, @JelleZijlstra?

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Comment thread Lib/test/test_typing.py Resolved

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@hmc-cs-mdrissi
Add comment for special forms test
378aaea
JelleZijlstra
JelleZijlstra reviewed Mar 1, 2024
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Thanks. I feel this should be treated as a new feature and not backported.

It's probably worth a versionchanged note in the typing docs, e.g. saying "ClassVar can now be nested inside Final".

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@carljm

carljm commented Mar 1, 2024

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@JelleZijlstra what do you see as the risk of backporting?

It seems like arguably a "bug" that (given the current de facto behavior of type checkers) there is no way to specify a Final, non-field, ClassVar, on a dataclass. Backporting this would fix that "bug" for all users of Python 3.11 and Python 3.12, rather than requiring they wait until 3.13.
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@JelleZijlstra

JelleZijlstra commented Mar 1, 2024

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It feels like a new feature to me; we're adding support for something that wasn't previously allowed. For example, if a library after the release of 3.12.3 starts using ClassVar[Final...] in its code and tests only on 3.12.3+, they would break all users who are still on 3.12.2 without realizing it.

Users on earlier versions can work around the problem by using quoted annotations.
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JelleZijlstra
JelleZijlstra approved these changes Mar 12, 2024
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@bedevere-app bedevere-app Bot added awaiting merge and removed awaiting review labels Mar 12, 2024
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JelleZijlstra merged commit d308d33 into python:main Mar 12, 2024
@bedevere-app bedevere-app Bot removed the awaiting merge label Mar 12, 2024
@AlexWaygood

AlexWaygood commented Mar 12, 2024

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It's probably worth a versionchanged note in the typing docs, e.g. saying "ClassVar can now be nested inside Final".

This still needs to be done
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@JelleZijlstra JelleZijlstra mentioned this pull request Mar 12, 2024
gh-89547: typing.rst: Add note about change in behavior with ClassVar/Final #116686
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@JelleZijlstra

JelleZijlstra commented Mar 12, 2024

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Thanks, done in #116686
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adorilson pushed a commit to adorilson/cpython that referenced this pull request Mar 25, 2024
@hmc-cs-mdrissi @adorilson
pythongh-89547: Support for nesting special forms like Final (python#…
a9334b8 
…116096)
@JWCS JWCS mentioned this pull request Mar 27, 2024
Allow Annotated to wrap Final and ClassVar python/mypy#12061
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diegorusso pushed a commit to diegorusso/cpython that referenced this pull request Apr 17, 2024
@hmc-cs-mdrissi @diegorusso
pythongh-89547: Support for nesting special forms like Final (python#…
3381773 
…116096)
@Viicos Viicos mentioned this pull request May 26, 2025
ClassVar[Final[T]] should be allowed microsoft/pyright#10500
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@JelleZijlstra JelleZijlstra JelleZijlstra approved these changes

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@hmc-cs-mdrissi @carljm @JelleZijlstra @AlexWaygood @Eclips4
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