False-positive errors when using type[T] (or Type[T]) in a PEP 604 type alias

@JelleZijlstra

Bug Report

The following two snippets (one using builtins.type, the other using typing.Type) both work fine at runtime on 3.10+. They should be accepted as valid type alias definitions on Python 3.10+ (and should be accepted in stub files regardless of Python version). However, mypy raises false-positive errors for both. The errors do not go away if you use typing.TypeAlias:

from typing import Type, TypeAlias

A = type[int] | str  # error: Value of type "Type[type]" is not indexable
B = Type[int] | str  # error: Unsupported left operand type for | ("object")
C: TypeAlias = type[int] | str  # error: Value of type "Type[type]" is not indexable
D: TypeAlias = Type[int] | str  # error: Unsupported left operand type for | ("object")

The order of the types in the union makes no difference when using type[T]: the following two variants still produce errors:

E = str | type[int]  # error: Value of type "Type[type]" is not indexable
F: TypeAlias = str | type[int]  # error: Value of type "Type[type]" is not indexable

However, the order of the types in the union does matter if you're using typing.Type[T]:

B = Type[int] | str  # error: Unsupported left operand type for | ("object")
D: TypeAlias = Type[int] | str  # error: Unsupported left operand type for | ("object")
G = str | Type[int]  # no error

Interestingly, the bug can only be reproduced if type or Type is the outermost object in the type alias. Neither of the following two snippets causes mypy errors:

E = list[type[int] | str]
F = type[int | str]

Mypy is also happy with type[T] or Type[T] being used in old-style typing.Union type aliases:

from typing import Union
G = Union[type[int], str]

To Reproduce

Mypy playground link here.

Expected Behavior

No error should be reported.

Cc. @JelleZijlstra

I have encountered the following

import typing as tp
from collections.abc import Coroutine

T = tp.TypeVar('T')

F: tp.TypeAlias = tp.Callable[[], T] | tp.Callable[[], Coroutine[None, None, T]]  # Unsupported left operand type for | ("object")
G: tp.TypeAlias = tp.Callable[[], T | Coroutine[None, None, T]]  # ok

It looks like the same issue?

@wrobell

@wrobell, I have another issue open for PEP-604 false-positives with respect to Callable: #12393. But I agree that these problems are linked.

This issue seems to exist for any runtime evaluation of type[X], not just in type aliases:

def f() -> type:
    return type[int]  # error: Value of type "Type[type]" is not indexable

This still happens when using the --python-version 3.10 option.

@JukkaL

Hm, on current master, two of them work, but other two (with lowercase type) fail with different error

tstalt.py:3: error: Value of type "Type[type]" is not indexable  [index]
tstalt.py:5: error: Value of type "Type[type]" is not indexable  [index]

cc @JukkaL who recently worked on this.

Fix issues with type aliases and new style unions

Fix issues with type aliases and new style unions (#14181)

Hi,

Thanks for working on mypy and the related tools!

Is this bug really fixed though? With the attached file (renamed with a .py extension) I get the following:

[roam@straylight ~/tmp/v/roam/woof]$ python3.10 --version
Python 3.10.9
[roam@straylight ~/tmp/v/roam/woof]$ python3.10 -m mypy --version
mypy 0.991 (compiled: yes)
[roam@straylight ~/tmp/v/roam/woof]$ python3.10 -m mypy --python-version=3.10 woof.py
woof.py:12: error: Value of type "Type[type]" is not indexable  [index]
Found 1 error in 1 file (checked 1 source file)
[roam@straylight ~/tmp/v/roam/woof]$ python3.10 woof.py
Handle IPv4Address
[roam@straylight ~/tmp/v/roam/woof]$ python3.10 woof.py some arguments here
Handle IPv6Address
[roam@straylight ~/tmp/v/roam/woof]$

If I use Union to define the IPClass alias, it works just fine. If I put the union - either using Union or the | operator - within the definition of the handle() function, it still works fine:

[roam@straylight ~/tmp/v/roam/woof]$ fgrep -e 'def handle' woof.py
def handle(ipcls: type[ipaddress.IPv4Address] | type[ipaddress.IPv6Address]) -> None:
[roam@straylight ~/tmp/v/roam/woof]$ python3.10 -m mypy --python-version=3.10 woof.py
Success: no issues found in 1 source file
[roam@straylight ~/tmp/v/roam/woof]$

woof.txt

@ppentchev

@ppentchev, there hasn't yet been a mypy release that includes the fix. Are you testing with the mypy master branch?

@AlexWaygood

@AlexWaygood Thanks for the lightning-fast reply! I had actually realized that, and I just came back to leave a "sorry for the noise, I just tested with the Git version and it works just fine!" comment... and you had already replied :)

Thanks again for all your work, and apologies!

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False-positive errors when using `type[T]` (or `Type[T]`) in a PEP 604 type alias #12392

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False-positive errors when using type[T] (or Type[T]) in a PEP 604 type alias #12392

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wrobell commented on Apr 12, 2022

AlexWaygood commented on Apr 12, 2022

mthuurne commented on May 31, 2022

ilevkivskyi commented on Nov 21, 2022

ppentchev commented on Jan 8, 2023

AlexWaygood commented on Jan 8, 2023

ppentchev commented on Jan 8, 2023

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False-positive errors when using `type[T]` (or `Type[T]`) in a PEP 604 type alias #12392