bpo-46475: Add typing.Never and typing.assert_never by JelleZijlstra · Pull Request #30842 · python/cpython · GitHub

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

Thank you! Awesome features!

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. :-( )

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

https://mail.python.org/archives/list/typing-sig@python.org/thread/CGIYKGMF6XLDX3F2JNRCCE7HC6K2XLZ2/ is the typing-sig thread. Nobody opposed it.

Should I wait for the tests or are they hanging?

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

Sorry, I've got more...

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

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.

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 :-).

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

LGTM — thanks!

Great! Thanks for being patient with me.

Thank you for the feedback!

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

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

86a868f

JelleZijlstra requested review from Fidget-Spinner and gvanrossum as code owners January 23, 2022 23:40

the-knights-who-say-ni added the CLA signed label Jan 23, 2022

bedevere-bot added the awaiting review label Jan 23, 2022

JelleZijlstra force-pushed the never branch from e3fe2cb to 86a868f Compare January 23, 2022 23:40

JelleZijlstra added the DO-NOT-MERGE label Jan 23, 2022

sobolevn reviewed Jan 24, 2022

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Comment thread Doc/library/typing.rst

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Comment thread Lib/typing.py

deduplicate tests

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Fidget-Spinner reviewed Jan 24, 2022

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davidfstr reviewed Jan 31, 2022

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JelleZijlstra added 3 commits January 30, 2022 18:20

Merge branch 'main' into never

403f8e5

Address David Foster's feedback

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Merge branch 'main' into never

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JelleZijlstra removed the DO-NOT-MERGE label Feb 2, 2022

gvanrossum reviewed Feb 2, 2022

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Comment thread Lib/test/test_typing.py Outdated

rename BottomTypeTests

1b297fe

JelleZijlstra closed this Feb 2, 2022

JelleZijlstra reopened this Feb 2, 2022

Merge branch 'main' into never

02a3e89

gvanrossum reviewed Feb 6, 2022

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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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AlexWaygood reviewed Feb 7, 2022

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Thanks Alex

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AlexWaygood reviewed Feb 7, 2022

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Comment thread Doc/library/typing.rst

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 Feb 8, 2022

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bedevere-bot added awaiting core review and removed awaiting review labels Feb 8, 2022

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Comment thread Lib/typing.py

antonagestam mentioned this pull request Feb 8, 2022

Exhaustiveness checking util python/typing#735

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gvanrossum approved these changes Feb 8, 2022

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bedevere-bot added awaiting merge and removed awaiting core review labels Feb 8, 2022

gvanrossum merged commit 243436f into python:main Feb 8, 2022

bedevere-bot removed the awaiting merge label Feb 8, 2022

JelleZijlstra deleted the never branch February 8, 2022 18:50

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

Merged

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

Merged

johnthagen mentioned this pull request Apr 18, 2022

Exhaustiveness Research johnthagen/sealed-typing-pep#4

Closed

JelleZijlstra mentioned this pull request Apr 19, 2022

typing.Never and typing.assert_never #90633

Closed

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.

+                      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.

bpo-46475: Add typing.Never and typing.assert_never by JelleZijlstra · Pull Request #30842 · python/cpython · GitHub

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bpo-46475: Add typing.Never and typing.assert_never - #30842

bpo-46475: Add typing.Never and typing.assert_never#30842gvanrossum merged 13 commits intopython:mainpython/cpython:mainfrom JelleZijlstra:neverJelleZijlstra/cpython:neverCopy head branch name to clipboard

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bpo-46475: Add typing.Never and typing.assert_never#30842
gvanrossum merged 13 commits into
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JelleZijlstra commented Jan 23, 2022 •
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