Description

JelleZijlstra

opened on Jan 22, 2022

Issue body actions

BPO	46475
Nosy	@gvanrossum, @davidfstr, @JelleZijlstra, @sobolevn, @Fidget-Spinner, @AlexWaygood
PRs	bpo-46475: Add typing.Never and typing.assert_never #30842

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/JelleZijlstra'
closed_at = <Date 2022-02-08.19:07:21.141>
created_at = <Date 2022-01-22.22:48:21.675>
labels = ['type-feature', 'library', '3.11']
title = 'typing.Never and typing.assert_never'
updated_at = <Date 2022-02-08.19:07:21.140>
user = 'https://github.com/JelleZijlstra'

bugs.python.org fields:

activity = <Date 2022-02-08.19:07:21.140>
actor = 'JelleZijlstra'
assignee = 'JelleZijlstra'
closed = True
closed_date = <Date 2022-02-08.19:07:21.141>
closer = 'JelleZijlstra'
components = ['Library (Lib)']
creation = <Date 2022-01-22.22:48:21.675>
creator = 'JelleZijlstra'
dependencies = []
files = []
hgrepos = []
issue_num = 46475
keywords = ['patch']
message_count = 5.0
messages = ['411297', '411301', '411309', '411347', '412860']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'David Foster', 'JelleZijlstra', 'sobolevn', 'kj', 'AlexWaygood']
pr_nums = ['30842']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue46475'
versions = ['Python 3.11']

Activity

1. JelleZijlstra commented on Jan 22, 2022

   

   JelleZijlstra

   on Jan 22, 2022

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   At the moment, https://docs.python.org/3.10/library/typing.html#typing.NoReturn simply says:

   Special type indicating that a function never returns.
   

   In practice, type checkers accept NoReturn as a bottom type in other positions too. We should document this behavior.
2. JelleZijlstra

   assigned

   JelleZijlstra

   on Jan 22, 2022
3. JelleZijlstra

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   3.9 (EOL)end of lifeend of life

   3.10 (EOL)end of lifeend of life

   3.11only security fixesonly security fixes

   on Jan 22, 2022
4. sobolevn commented on Jan 22, 2022

   

   sobolevn

   on Jan 22, 2022

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   I agree.

   Plus, I cannot refrain from saying that it is generally quite hard to teach what "NoReturn" is.

   I start by explaining what bottom type and void is and then just say that in Python it is called NoReturn. I remember that there was a discussion about Never alias for it.

   Jelle, will you please push it? :)
5. AlexWaygood commented on Jan 22, 2022

   

   AlexWaygood

   on Jan 22, 2022

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   I also agree that documenting this is a great idea. It's already been adopted by large swathes of the typing community for use in this context. The value of having NoReturn act as a "bottom type" is evident. Time to make it official, in my opinion.
6. JelleZijlstra commented on Jan 23, 2022

   

   JelleZijlstra

   on Jan 23, 2022

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   Based on feedback here and on typing-sig I'm now leaning towards adding typing.Never along with typing.assert_never. I'll submit a single patch for these as the documentation will be closely linked.
7. JelleZijlstra

   removed

   3.9 (EOL)end of lifeend of life

   on Jan 23, 2022
8. 7 remaining items

9. gvanrossum commented on Feb 8, 2022

   

   gvanrossum

   on Feb 8, 2022

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   New changeset 243436f by Jelle Zijlstra in branch 'main':
   bpo-46475: Add typing.Never and typing.assert_never (GH-30842)
   243436f
10. JelleZijlstra

    closed this as completedon Feb 8, 2022
11. JelleZijlstra

    closed this as completedon Feb 8, 2022
12. ezio-melotti

    transferred this issue fromon Apr 10, 2022
13. ptmcg commented on Apr 19, 2022

    

    ptmcg

    on Apr 19, 2022

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    What is the purpose of the required argument to assert_never()? Regardless of what I pass to this function, the raised exception is "AssertionError: Expected code to be unreachable"
14. JelleZijlstra commented on Apr 19, 2022

    

    JelleZijlstra

    on Apr 19, 2022

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    @ptmcg it always throws at runtime; the point is that static typecheckers will verify that the code really is unreachable. See https://docs.python.org/3.11/library/typing.html#typing.assert_never and https://typing.readthedocs.io/en/latest/source/unreachable.html
15. gvanrossum commented on Apr 19, 2022

    

    gvanrossum

    on Apr 19, 2022

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    IIUC the question is why does it have an argument and what effect does it have? (It seems to have none, then why is it required?)
16. ptmcg commented on Apr 19, 2022

    

    ptmcg

    on Apr 19, 2022

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    Thank you @gvanrossum , that is what I was asking.
17. JelleZijlstra commented on Apr 19, 2022

    

    JelleZijlstra

    on Apr 19, 2022

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    I see. Usually you'd use this function in a place where some variable gets narrowed down to Never. Making it an explicit argument makes it possible to implement the function without any special casing in type checkers, because they already know how to treat Never in an argument position.

    We could add a similar function with no arguments that asserts a block is unreachable, but unlike assert_never() it would need to be special cased in type checkers.

    We might want to revisit the fact that the error message is always the same. We did that on the basis of security concerns about str() on untrusted input, but I don't find those concerns that convincing, and it's useful to have some representation of the object in the error message.
18. gvanrossum commented on Apr 19, 2022

    

    gvanrossum

    on Apr 19, 2022

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    Security concerns? That seems excessive. Millions of things in the stdlib print the repr() of the thing that unexpectedly failed some check.

    Reacted by Alex Waygood
19. JelleZijlstra commented on Apr 20, 2022

    

    JelleZijlstra

    on Apr 20, 2022

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    Sending a PR shortly to add the asserted value to assert_never()'s error message, and also to link to https://typing.readthedocs.io/en/latest/source/unreachable.html while I'm at it.
20. JelleZijlstra

    reopened this on Apr 20, 2022
21. JelleZijlstra

    added a commit that references this issue on Apr 20, 2022

    pythongh-90633: Improve error and docs for typing.assert_never

    ce3da53
22. JelleZijlstra

    closed this as completedin 93d2801on Apr 25, 2022
23. serhiy-storchaka

    added a commit that references this issue on Aug 9, 2026

    pythongh-90633: Improve error and docs for typing.assert_never (pytho…

    93d2801

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