Unexpected TypeError with type alias+issubclass+ABC

[AMDmi3]

BPO	45326
Nosy	@gvanrossum, @corona10, @AMDmi3, @Fidget-Spinner, @akulakov, @burritoatspoton

^{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 = None
created_at = <Date 2021-09-29.21:14:09.774>
labels = ['type-bug', 'library', '3.9', '3.10', '3.11']
title = 'Unexpected TypeError with type alias+issubclass+ABC'
updated_at = <Date 2022-02-28.01:52:34.626>
user = 'https://github.com/AMDmi3'

bugs.python.org fields:

activity = <Date 2022-02-28.01:52:34.626>
actor = 'andrei.avk'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-09-29.21:14:09.774>
creator = 'AMDmi3'
dependencies = []
files = []
hgrepos = []
issue_num = 45326
keywords = []
message_count = 2.0
messages = ['402914', '414170']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'corona10', 'AMDmi3', 'kj', 'andrei.avk', 'burrito']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45326'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[AMDmi3]

Here's a curious problem. issubclass() check of a type against an ABC-derived class raises TypeError claiming that type is not a class, however inspect.isclass() says it's a class, and issubclass() check against a simple class works fine:

from abc import ABC

class C1:
    pass

issubclass(dict[str, str], C1)  # False

class C2(ABC):
    pass

issubclass(dict[str, str], C2)  # TypeError: issubclass() arg 1 must be a class

I've ran into this problem while using inspect module to look for subclasses of a specific ABC in a module which may also contain type aliases, and after converting a type alias from Dict[str, str] to modern dict[str, str] I've got an unexpected crash in this code:

    if inspect.isclass(member) and issubclass(member, superclass):

Not sure which is the culprit, ABC or how dict[]-style type aliases are implemented.

[akulakov]

This error was added in https://bugs.python.org/issue33018 . See some discussion on that issue.

Note that first arg needs to be a type (i.e. instance of type) to avoid this error:

[ins] In [41]: class C(ABC):0

[ins] In [42]: issubclass(dict, C)
Out[42]: False

[ins] In [43]: issubclass('', C) # TypeError: issubclass() arg 1 must be a class

[ins] In [44]: issubclass(typing.Dict, C) # same error as above

[sillydan1]

I encountered a slight variation of this issue. I am using python 3.10.12:

import inspect
from abc import ABCMeta

class C1(ABCMeta):
    pass

class C2(metaclass=C1):
    pass

DictAlias = dict[str,str]

print(inspect.isclass(DictAlias)) # True
print(issubclass(DictAlias, C1)) # False
print(issubclass(DictAlias, C2)) # TypeError: issubclass() arg 1 must be a class

Note that the original example by AMDmi3 also fails