Checking Generic Type at Runtime #13053

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1. samuelstevens

   added

   bugmypy got something wrongmypy got something wrong

   on Jul 1, 2022
2. AlexWaygood

   added

   topic-type-variables

   on Jul 1, 2022
3. erictraut commented on Jul 1, 2022

   

   erictraut

   on Jul 1, 2022

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   This behavior is correct because isinstance(obj, int) does not guarantee that obj is an int; it simply guarantees that it's a subtype of int. However, the __add__ method for int always returns an int, not a subtype of int.

   class IntSubclass(int): ...
   
   i1 = IntSubclass(0)
   i2 = increment(i1)
   
   # At runtime, i2 is an instance of 'int', not 'IntSubclass'.
   print(type(i2)) # <class 'int'>

   👍React with 👍2Reacted by Alex Waygood and Eloff
4. samuelstevens commented on Jul 2, 2022

   

   samuelstevens

   on Jul 2, 2022

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   That makes sense. Thank you for explainging!

   My actual use case is more complicated, however. Instead of int, I have some objects of a class that call some methods. Something like:

   from typing import TypeVar
   
   T = TypeVar("T")
   
   
   class Dummy:
       def __init__(self):
           self.state: int = 0
   
       def mutate_state(self):
           self.state += 1
   
   
   def handle(obj: T) -> T:
       if isinstance(obj, Dummy):
           obj.mutate_state()
           return obj
       else:
           return obj

   I get the same error:

   $ mypy --strict scratch.py
   scratch.py:17: error: Incompatible return value type (got "Dummy", expected "T")
   Found 1 error in 1 file (checked 1 source file)
   

   Even though obj might just be a subclass of Dummy, I am still returning obj directly which is still of type T. So shouldn't this be error-free?
5. erictraut commented on Jul 2, 2022

   

   erictraut

   on Jul 2, 2022

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   Yes, your revised sample is type safe. I don't think mypy should emit an error in this case. By comparison, pyright does not.

   Here's a workaround for mypy:

   @overload
   def handle(obj: Dummy) -> Dummy: ...
   @overload
   def handle(obj: T) -> T: ...
   
   def handle(obj: T | Dummy) -> T | Dummy:
       if isinstance(obj, Dummy):
           obj.mutate_state()
           return obj
       else:
           return obj
6. samuelstevens commented on Jul 2, 2022

   

   samuelstevens

   on Jul 2, 2022

   Last edited by samuelstevens

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   Thanks for the suggestion; I will try it in my full case but I was hoping to avoid all the extra boilerplate! (I have half a dozen classes that have special isinstance cases.)

   I'll leave this open as a bug in mypy. If I have time, maybe I will be able to look into why this happens (probably not).
7. ilevkivskyi commented on May 31, 2025

   

   ilevkivskyi

   on May 31, 2025

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   Yeah, this is not a bug. The revised example is going to be fixed in #19183
8. ilevkivskyi

   closed this as not plannedon May 31, 2025