Support GenericAlias in typing

[serhiy-storchaka]

BPO	40396
Nosy	@gvanrossum, @serhiy-storchaka, @ilevkivskyi
PRs	bpo-40396: Support GenericAlias in the typing functions. #19718

^{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 2020-04-26.19:27:57.404>
created_at = <Date 2020-04-26.16:02:57.099>
labels = ['type-feature', 'library', '3.9']
title = 'Support GenericAlias in typing'
updated_at = <Date 2020-04-26.19:27:57.404>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2020-04-26.19:27:57.404>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2020-04-26.19:27:57.404>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2020-04-26.16:02:57.099>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 40396
keywords = ['patch']
message_count = 2.0
messages = ['367308', '367326']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'serhiy.storchaka', 'levkivskyi']
pr_nums = ['19718']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40396'
versions = ['Python 3.9']

[serhiy-storchaka]

Currently typing functions get_origin(), get_args() and get_type_hints() do not support GenericAlias.

>>> from typing import *
>>> get_origin(List[int])
<class 'list'>
>>> get_origin(list[int])
>>> get_args(List[int])
(<class 'int'>,)
>>> get_args(list[int])
()
>>> def foo(x: List[ForwardRef('X')], y: list[ForwardRef('X')]) -> None: ... 
... 
>>> class X: ...
... 
>>> get_type_hints(foo)
{'x': typing.List[__main__.X], 'y': list[ForwardRef('X')], 'return': <class 'NoneType'>}

The proposed PR fixes this.

[serhiy-storchaka]

New changeset 68b352a by Serhiy Storchaka in branch 'master':
bpo-40396: Support GenericAlias in the typing functions. (GH-19718)
68b352a

[danielkatzan]

@serhiy-storchaka
why does get_origin(list) returns None while get_origin(List) returns <class 'list'>

shouldn't list behaves exactly the same as List? (the same question applies to all generics)

[gvanrossum]

@danielkatzan Looks like an omission in the code for get_origin(). Would you like to submit a PR to fix it? I can't promise that it will make it into 3.11 but it will be in 3.11.1.

[danielkatzan]

@gvanrossum Sure, I would be happy to, but could use a little pointer here
Not sure how this can be accomplished

note while type(list[str]) is <class 'types.GenericAlias'> and has the __origin__ attribute, type(list) is actually <class 'type'> and doesn't have an __origin__ attribute

I can add an explicit if, checking if the tp is actually dict\list etc, and return that instead of None

will that be a valid approach? or just masking a deeper issue that might pop up later in a different location?