Base class attribute type does not provide type context in subclass #4547

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1. JukkaL

   added

   bugmypy got something wrongmypy got something wrong

   priority-1-normal

   topic-usability

   on Feb 8, 2018
2. ilevkivskyi commented on Mar 6, 2019

   

   ilevkivskyi

   on Mar 6, 2019

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   #6511 provides another use case for this.
3. ashb commented on Mar 6, 2019

   

   ashb

   on Mar 6, 2019

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   Minimal repro case:

   from typing import Iterable
   
   class A:
       a: Iterable[str] = ("a", )
   
   class B(A):
       a = ("a", "b")
   
   class C(B):
       a = ("a", "b", "c")

   And the error:

   a.py:10: error: Incompatible types in assignment (expression has type "Tuple[str, str, str]", base class "B" defined the type as "Tuple[str, str]")

   The error is reported on the assignment in C.

   I would have expected that B.a would have the same type as A.a since B.a doesn't have an explicit type.

   (This is the case from #6511 copied here in case it is useful)
4. ilevkivskyi commented on May 3, 2019

   

   ilevkivskyi

   on May 3, 2019

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   (Raising priority to high since this keeps coming.)
5. ilevkivskyi

   added

   priority-0-high

   and removed

   priority-1-normal

   on May 3, 2019
6. onlined commented on May 13, 2019

   

   onlined

   on May 13, 2019

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   This example from #6766 (comment), which is related to partial list type, doesn't give an error while it should:

   class A:
       x = ['a', 'b']
   
   class B(A):
       x = []
       x.append(2)
7. AlexWaygood

   added

   topic-inheritanceInheritance and incompatible overridesInheritance and incompatible overrides

   topic-type-contextType context / bidirectional inferenceType context / bidirectional inference

   on Apr 3, 2022
8. ilevkivskyi commented on Aug 23, 2022

   

   ilevkivskyi

   on Aug 23, 2022

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   It looks like the situation is even worse here. Here we have no partial types and we still infer wrong type (the one that violates Liskov):

   from typing import List, Optional
   
   class B:
       x: List[Optional[int]]
   
   class C(B):
       x = [1]
   
   reveal_type(C().x)  # Revealed type is "builtins.list[builtins.int]"

   The problem is that we use supertype as type context when checking compatibility. But then again when storing the variable type we use empty context.
9. hauntsaninja

   closed this as completedin #13494on Aug 24, 2022