Incompatible return value type with TypeVar

[marouane-miftah]

mypy=0.800
python=3.8.6

The following code generates
error: Incompatible return value type (got "str", expected "Union[ndarray, T]")

from __future__ import annotations
from typing import TypeVar, Union
import numpy

T = TypeVar("T")

def _test(a: list[T]) -> Union[numpy.ndarray, T]:
 if isinstance(a[0], str):
 return a[0]
 return numpy.array(a)

[giladreti]

Same here.

This code generates Incompatible return value type (got "int", expected "T"):

from __future__ import annotations

from typing import Type, TypeVar

T = TypeVar("T", bound=int)


def test(cls: Type[T]) -> T:
 if cls is int:
 return int()

 return cls()

[aucampia]

Also having this problem:

from typing import TypeVar
import unittest

AnyT = TypeVar("AnyT")


def strip_if_str(value: AnyT) -> AnyT:
 if isinstance(value, str):
 return value.strip()
 return value


class TestStripIfStr(unittest.TestCase):
 def test_str(self) -> None:
 self.assertEqual(strip_if_str(" a b c "), "a b c")

 def test_notstr(self) -> None:
 self.assertEqual(strip_if_str(3), 3)


if __name__ == "__main__":
 unittest.main()

$ python -m mypy --show-error-context --show-error-codes .
test_typevar_identity.py: note: In function "strip_if_str":
test_typevar_identity.py:9: error: Incompatible return value type (got "str", expected "AnyT") [return-value]
Found 1 error in 1 file (checked 1 source file)

$ python -m unittest
..
----------------------------------------------------------------------
Ran 2 tests in 0.000s

OK
$ python --version
Python 3.10.1
$ python -m mypy --version
mypy 0.931

I think this is similar to:

• TypeVar incompatible with constrained union #9424

[aucampia]

Possible other issues that is related to this one:

• TypeVar incompatible with constrained union #9424
• False-positive on TypeVar bound class return value #10817 (comment)
• the possibility that isinstance(some_var, (A, B)) causes mypy to forget about a some_var's type #10835

[fzimmermann89]

Are there any plans to fix this? Or is there a good work around?