typing.get_args(Callable) fails

[serhiy-storchaka]

BPO	40398
Nosy	@gvanrossum, @serhiy-storchaka, @ilevkivskyi
PRs	bpo-40398: Fix typing.get_args() for special generic aliases. #19720 [3.8] bpo-40398: Fix typing.get_args() for special generic aliases. (GH-19720) #19857

^{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 2020-05-02.08:08:24.496>
created_at = <Date 2020-04-26.16:46:07.424>
labels = ['3.8', 'type-bug', 'library', '3.9']
title = 'typing.get_args(Callable) fails'
updated_at = <Date 2020-05-02.08:08:24.495>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2020-05-02.08:08:24.495>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2020-05-02.08:08:24.496>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2020-04-26.16:46:07.424>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 40398
keywords = ['patch']
message_count = 10.0
messages = ['367313', '367322', '367327', '367328', '367329', '367331', '367405', '367898', '367919', '367920']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'serhiy.storchaka', 'levkivskyi']
pr_nums = ['19720', '19857']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40398'
versions = ['Python 3.8', 'Python 3.9']

[serhiy-storchaka]

>>> from typing import *
>>> get_args(Callable)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/typing.py", line 1412, in get_args
    if get_origin(tp) is collections.abc.Callable and res[0] is not Ellipsis:
IndexError: tuple index out of range

What is the correct behavior?

[gvanrossum]

Maybe it could return (Ellipsis, Any) in that case. The most general form of Callable is Callable[..., Any] and that's that get_args() returns for that.

[ilevkivskyi]

I think it should return empty tuple: First, for consistency with other things like get_args(Tuple) == () and get_args(List) == (). Second, although Callable is equivalent to Callable[..., Any] in the static world, authors of some runtime checkers might want to distinguish whether a user wrote one or another.

[gvanrossum]

Agreed, that's better!

[serhiy-storchaka]

get_args(List) == (T,)

[ilevkivskyi]

Oh my, this looks like another bug. I don't have Python 3.8 at hand so just tried typing_inspect on Python 3.6. I think this may be caused by the "double use" of _GenericAlias for which you opened your WIP PR.

[serhiy-storchaka]

New changeset 6292be7 by Serhiy Storchaka in branch 'master':
bpo-40398: Fix typing.get_args() for special generic aliases. (GH-19720)
6292be7