typing.Never and typing.assert_never

[JelleZijlstra]

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']

[JelleZijlstra]

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.

[sobolevn]

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? :)

[AlexWaygood]

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.

[JelleZijlstra]

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.