Problems caused by isinstance(list[int], type) returning True

[serhiy-storchaka]

BPO	45665
Nosy	@gvanrossum, @serhiy-storchaka, @wyfo, @Fidget-Spinner, @AlexWaygood, @martinitus
Dependencies	bpo-40296: help(list[int]) fails bpo-45438: inspect not capturing type annotations created by class_getitem bpo-45662: Incorrect repr of InitVar of a type alias bpo-45663: is_dataclass() does not work for dataclasses which are subclasses of types.GenericAlias bpo-45664: resolve_bases() and new_class() do not work with type alias of a built-in type bpo-46032: functools' singledispatch does not support GenericAlias

^{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 2021-10-29.08:27:43.950>
labels = ['type-bug', 'library', '3.9', '3.10', '3.11']
title = 'Problems caused by isinstance(list[int], type) returning True'
updated_at = <Date 2022-01-06.16:58:47.655>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2022-01-06.16:58:47.655>
actor = 'AlexWaygood'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-10-29.08:27:43.950>
creator = 'serhiy.storchaka'
dependencies = ['40296', '45438', '45662', '45663', '45664', '46032']
files = []
hgrepos = []
issue_num = 45665
keywords = []
message_count = 17.0
messages = ['405290', '405291', '405427', '405429', '408207', '409167', '409233', '409679', '409691', '409694', '409697', '409701', '409702', '409703', '409711', '409792', '409881']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'serhiy.storchaka', 'joperez', 'kj', 'AlexWaygood', 'martinitus']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45665'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

• help(list[int]) fails #84476
• inspect not capturing type annotations created by __class_getitem__ #89601
• Incorrect repr of InitVar of a type alias #89825
• is_dataclass() does not work for dataclasses which are subclasses of types.GenericAlias #89826
• resolve_bases() and new_class() do not work with type alias of a built-in type #89827
• functools' singledispatch does not support GenericAlias #90190
• Enum does not work if member is an instance of types.GenericAlias #93847

[serhiy-storchaka]

This is a meta-issue for problems caused by isinstance(list[int]) returning True.

See also discussion in bpo-45438.

[serhiy-storchaka]

isinstance(x, type) returns True for instances of types.GenericAlias (like list[int]). While it may help in some cases related to typing, in many unrelated cases it causes problems if the value which is not a type passes checks for types.

Also, isinstance(x, type) been not equal to issubclass(type(x), type) can cause other problems. No matter what the result should be, it should be consistent.

There are many occurrences of isinstance(..., type) in the code.

$ find Lib -name '*.py' \! -path '*/test*' -exec egrep 'isinstance.*,
type\)' '{}' + | wc -l
55

And all of them can potentially be broken if pass a types.GenericAlias instance. Many of them are safe, but every case should be analyzed.

[martinitus]

One thing that probably should be considered in this context:

isinstance(arg, type) == issubclass(type(arg), type)

Holds True for arg in (Optional[X], Union[X, Y]). Both sides evaluate to False. (v3.10.0)

While I still think both sides evaluating to True would be more intuitive, this supports the proposed change.

Small test snippet:

from typing import Dict, List, Set, Tuple, Optional, Union

import pytest


@pytest.mark.parametrize('arg', [
    list, List[int], list[int],
    dict, Dict[str, int], dict[str, int],
    set, Set[int], set[int],
    tuple, Tuple[str, int], tuple[str, int],
    Optional[int],
    Union[int, str]
])
def test_invariant(arg):
    same = isinstance(arg, type) == issubclass(type(arg), type)
    result = "Check" if same else "Failed"
    print(f"\n{result}: Testing: {arg=} with {type(arg)=}: {isinstance(arg, type)=} <> {issubclass(type(arg), type)=}")
    assert same

Any other commonly used annotations that could be added to the checklist?