typing: allow Annotated in outermost scope

[GBeauregard]

BPO	46491
Nosy	@gvanrossum, @JelleZijlstra, @asottile, @miss-islington, @Fidget-Spinner, @GBeauregard
PRs	bpo-46491: Allow Annotated on outside of Final/ClassVar #30864 [3.10] bpo-46491: Allow Annotated on outside of Final/ClassVar (GH-30864) #30873 [3.9] bpo-46491: Allow Annotated on outside of Final/ClassVar (GH-30864) #30874

^{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 = None
closed_at = <Date 2022-01-25.14:40:07.659>
created_at = <Date 2022-01-23.19:03:20.373>
labels = ['type-bug', 'library', '3.9', '3.10', '3.11']
title = 'typing: allow Annotated in outermost scope'
updated_at = <Date 2022-01-26.00:26:54.760>
user = 'https://github.com/GBeauregard'

bugs.python.org fields:

activity = <Date 2022-01-26.00:26:54.760>
actor = 'gvanrossum'
assignee = 'none'
closed = True
closed_date = <Date 2022-01-25.14:40:07.659>
closer = 'kj'
components = ['Library (Lib)']
creation = <Date 2022-01-23.19:03:20.373>
creator = 'GBeauregard'
dependencies = []
files = []
hgrepos = []
issue_num = 46491
keywords = ['patch']
message_count = 13.0
messages = ['411404', '411405', '411418', '411557', '411598', '411599', '411600', '411686', '411688', '411690', '411691', '411692', '411693']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'JelleZijlstra', 'Anthony Sottile', 'miss-islington', 'kj', 'GBeauregard']
pr_nums = ['30864', '30873', '30874']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46491'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[GBeauregard]

Currently, typing.Annotated (PEP-593) cannot be used at runtime with typing.Final and typing.ClassVar with Annotated on the outside:

from typing import Annotated, Final
# TypeError: typing.Final[int] is not valid as type argument
var: Annotated[Final[int], "foo"] = 4

The only tenuously related mention of this I can find in a PEP is in PEP-593 (Annotated) which states "The first argument to Annotated must be a valid type".

I believe the runtime behavior should be changed to allow any ordering for Annotated with ClassVar and Final. This was discussed in the typing-sig PEP-655 thread (TypedDict Required and NotRequired) where the current plan is to allow Required/NotRequired in any nesting order with Annotated while suggesting the ClassVar/Final ordering restriction be lifted: https://mail.python.org/archives/list/typing-sig@python.org/message/22CJ5TJGIJ563D6ZKB7R3VUZXTZQND5X/

The argument for doing so is on the mailing list: https://mail.python.org/archives/list/typing-sig@python.org/message/MPMOIBX3XFXCD4ZNDC6AV4CLSI5LN544/

To summarize: adopting an overly strict view of what constitutes a valid type for Annotated creates difficulties for people who use runtime introspection of Annotated annotations by requiring them to parse additional typing or field annotations (https://bugs.python.org/msg411067). This needlessly exacerbates tension between typing and non-typing uses of annotation space. In order to be a good citizen to other annotation users, the Annotated runtime ordering restriction should be lifted.

[JelleZijlstra]

I support making this change. It looks like the simplest implementation is simply to remove the mention of ClassVar and Final in typing._type_check.

[gvanrossum]

+1--
--Guido (mobile)

[miss-islington]

New changeset e1abffc by Gregory Beauregard in branch 'main':
bpo-46491: Allow Annotated on outside of Final/ClassVar (GH-30864)
e1abffc

[miss-islington]

New changeset 41e0aea by Miss Islington (bot) in branch '3.10':
bpo-46491: Allow Annotated on outside of Final/ClassVar (GH-30864)
41e0aea