collections.abc.Callable and type variables

[serhiy-storchaka]

BPO	40494
Nosy	@gvanrossum, @serhiy-storchaka, @ilevkivskyi, @Fidget-Spinner

^{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 2020-12-25.22:10:21.763>
created_at = <Date 2020-05-04.08:07:12.472>
labels = ['type-bug', 'library', '3.9']
title = 'collections.abc.Callable and type variables'
updated_at = <Date 2020-12-25.22:10:21.762>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2020-12-25.22:10:21.762>
actor = 'gvanrossum'
assignee = 'none'
closed = True
closed_date = <Date 2020-12-25.22:10:21.763>
closer = 'gvanrossum'
components = ['Library (Lib)']
creation = <Date 2020-05-04.08:07:12.472>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 40494
keywords = []
message_count = 8.0
messages = ['368015', '368020', '368055', '368058', '368314', '381070', '383744', '383772']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'serhiy.storchaka', 'levkivskyi', 'kj']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40494'
versions = ['Python 3.9']

[serhiy-storchaka]

There is a difference between typing.Callable and collections.abc.Callable.

>>> import typing, collections.abc
>>> T = typing.TypeVar('T')
>>> C1 = typing.Callable[[T], T]
>>> C2 = collections.abc.Callable[[T], T]
>>> C1
typing.Callable[[~T], ~T]
>>> C2
collections.abc.Callable[[~T], ~T]
>>> C1[int]
typing.Callable[[int], int]
>>> C2[int]
collections.abc.Callable[[~T], int]

[ilevkivskyi]

Here I think the behavior of typing.Callable is correct.

[gvanrossum]

Yeah, the fix will require a variant of types.GenericAlias that substitute's type variables in lists.

[serhiy-storchaka]

Of course. There is more than one way to fix it:

• Make GenericAlias substituting type variables in list. It is easier and it will fix this particular case, but there will be subtle differences in __args__.
• Add a GenericAlias subclass with overridden constructor, __repr__, __getitem__, __reduce__ (like the _GenericAlias subclass added in bpo-40397).

I think we should first resolve bpo-40397 and later decide what way be better. It is not necessary to reproduce all details of _GenericAlias if the practical behavior is the same.

[gvanrossum]

Hm, I am indeed torn. ISTM a subclass just for Callable is slightly better.

[gvanrossum]

From https://bugs.python.org/issue42195 it looks like we need to create a subclass just for Callable. See https://bugs.python.org/issue42102 and PR 22848.

[Fidget-Spinner]

Now that bpo-42195 has been resolved by subclassing types.GenericAlias, can this be closed?

On 3.9 and 3.10:

>>> import typing, collections.abc
>>> T = typing.TypeVar('T')
>>> C2 = collections.abc.Callable[[T], T]
>>> C2[int]
collections.abc.Callable[[int], int]

It seems to be fixed :).

[gvanrossum]

Indeed. Thanks!