Incompatible return value type with TypeVar #10003

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1. marouane-miftah

   added

   bugmypy got something wrongmypy got something wrong

   on Feb 1, 2021
2. giladreti commented on Feb 1, 2021

   

   giladreti

   on Feb 1, 2021

   Last edited by giladreti

   More actions

   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()

   👍React with 👍2Reacted by Mo Abrahams and Nick Khitrov
3. aucampia commented on Jan 22, 2022

   

   aucampia

   on Jan 22, 2022

   Last edited by aucampia

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   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
4. aucampia commented on Jan 22, 2022

   

   aucampia

   on Jan 22, 2022

   Last edited by aucampia

   More actions

   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
5. fzimmermann89 commented on Nov 10, 2024

   

   fzimmermann89

   on Nov 10, 2024

   More actions

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

   closed this as completedin #19183on Jun 3, 2025
7. ilevkivskyi

   closed this as completedin 5a0fa55on Jun 3, 2025
8. BobTheBuidler

   added a commit that references this issue on Jun 3, 2025

   Narrow type variable bounds in binder (python#19183)

   ...

   5a0fa55