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GeezerVison is showing github.com — bpo-46475: Add typing.Never and typing.assert_never by JelleZijlstra · Pull Request #30842 · python/cpython

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JelleZijlstra commented Jan 23, 2022 •

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Another pending proposal from typing-sig.

Add a Never primitive that is semantically identical to NoReturn, but documented explicitly as a bottom type. I chose to make it a separate object instead of a pure alias for NoReturn so that dynamic checkers can distinguish between the two and so that we can document them separately.
Add an assert_never() function to statically assert that code is unreachable. Based on @davidfstr's feedback, the error raised by the function does not include the argument.

https://bugs.python.org/issue46475

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          bpo-46475: Add typing.Never and typing.assert_never

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JelleZijlstra requested review from Fidget-Spinner and gvanrossum as code owners

January 23, 2022 23:40

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Jan 23, 2022

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Jan 23, 2022

JelleZijlstra force-pushed the never branch from e3fe2cb to 86a868f Compare

January 23, 2022 23:40

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Jan 23, 2022

sobolevn reviewed

Jan 24, 2022

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Thank you! Awesome features!

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          deduplicate tests

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JelleZijlstra added 3 commits

January 30, 2022 18:20


          Merge branch 'main' into never

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          Address David Foster's feedback

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          Merge branch 'main' into never

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I think we should iterate this until @Fidget-Spinner approves it and then he can merge it. I think the idea is good, and IIRC typing-sig is in agreement. (Though are there folks at Pyre or Google who don't read it but want issues instead? Sigh. :-( )

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          rename BottomTypeTests

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JelleZijlstra commented Feb 2, 2022

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Thanks @gvanrossum! I think I addressed all previous comments, so from my perspective this is ready.

https://mail.python.org/archives/list/[email protected]/thread/CGIYKGMF6XLDX3F2JNRCCE7HC6K2XLZ2/ is the typing-sig thread. Nobody opposed it.

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JelleZijlstra closed this

Feb 2, 2022

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          Merge branch 'main' into never

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gvanrossum commented Feb 6, 2022

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Should I wait for the tests or are they hanging?

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JelleZijlstra commented Feb 6, 2022

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They just started, I had to fix a merge conflict. It's going to conflict with the Self change too, which is why I pinged you on that one first. I think this is the easiest way forward:

We wait for the tests on Self to pass and merge it
I fix the conflict here and wait for tests to pass again
Then we merge this one

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gvanrossum reviewed

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Sorry, I've got more...

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		def int_or_str(arg: int \| str) -> None:
		match arg:
		case int():
		print("It's an int")
		case str():
		print("It's a str")
		case _:
		assert_never(arg)

		If a type checker finds that a call to assert_never() is
		reachable, it will emit an error.

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This example is much better than the examples in the main doc. :-) And so is the sentence following the example.

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JelleZijlstra Feb 7, 2022

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That sentence is also in the docs for assert_never, but not Never. I'm pushing some changes to improve the Never docs according to your feedback.

Honestly I'm a bit confused about the relationship between the docstring and the RST docs. I end up copying from one to the other.

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gvanrossum Feb 8, 2022

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Usually the docstring is shorter, since it is often displayed by tools that have limited output space (it's really annoying to get a scroll bar in a tooltip in VS Code :-).

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JelleZijlstra added 2 commits

February 6, 2022 16:37


          Guido's feedback

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          Remove another "Notation for"

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JelleZijlstra commented Feb 7, 2022

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Thanks for your feedback! I pushed a round of docs changes.

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AlexWaygood reviewed

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          Thanks Alex

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JelleZijlstra added 3 commits

February 7, 2022 19:35


          remove extra example

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          Merge branch 'main' into never

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          raise AssertionError

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JelleZijlstra requested a review from gvanrossum

February 8, 2022 05:18

AlexWaygood approved these changes

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LGTM — thanks!

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antonagestam mentioned this pull request

Feb 8, 2022

Exhaustiveness checking util python/typing#735

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Great! Thanks for being patient with me.

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bedevere-bot added awaiting merge and removed awaiting core review labels

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gvanrossum merged commit 243436f into python:main

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JelleZijlstra deleted the never branch

February 8, 2022 18:50

JelleZijlstra commented Feb 8, 2022

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Thank you for the feedback!

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JelleZijlstra added a commit to JelleZijlstra/typing that referenced this pull request

Feb 9, 2022


          Add Never and assert_never

2faa2a8

Backport of python/cpython#30842, with additional tests from @sobolevn's python/cpython#31222.

This was referenced Feb 9, 2022

Add Never and assert_never python/typing#1060

Merged

add new typing features python/typeshed#7169

Merged

Add support for Never python/mypy#12153

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JelleZijlstra added a commit to python/typing that referenced this pull request

Feb 11, 2022


          Add Never and assert_never (#1060)

a53957c

Backport of python/cpython#30842, with additional tests from @sobolevn's python/cpython#31222.

henryiii mentioned this pull request

Feb 14, 2022

refactor: identifiers in a file pypa/cibuildwheel#511

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johnthagen mentioned this pull request

Apr 18, 2022

Exhaustiveness Research johnthagen/sealed-typing-pep#4

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JelleZijlstra mentioned this pull request

Apr 19, 2022

typing.Never and typing.assert_never #90633

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JelleZijlstra added a commit to python/typing_extensions that referenced this pull request

May 19, 2022


          Add Never and assert_never (#1060)

5054b4b

Backport of python/cpython#30842, with additional tests from @sobolevn's python/cpython#31222.

JelleZijlstra added a commit to python/typing_extensions that referenced this pull request

May 19, 2022


          Add Never and assert_never (#1060)

039b433

Backport of python/cpython#30842, with additional tests from @sobolevn's python/cpython#31222.

JelleZijlstra added a commit to python/typing_extensions that referenced this pull request

May 19, 2022


          Add Never and assert_never (#1060)

ba258bb

Backport of python/cpython#30842, with additional tests from @sobolevn's python/cpython#31222.

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JelleZijlstra commented Jan 23, 2022 • edited by bedevere-bot Loading Uh oh! There was an error while loading. Please reload this page.

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JelleZijlstra commented Jan 23, 2022 •

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