Cannot infer type of generic attributes in match statements when inheritance is involved #13620

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1. ariebovenberg

   added

   bugmypy got something wrongmypy got something wrong

   on Sep 7, 2022
2. ariebovenberg

   changed the title [-]Cannot infer type of attributes in `match` statements when inheritance is involved[/-] [+]Cannot infer type of generic attributes in `match` statements when inheritance is involved[/+] on Sep 7, 2022
3. sobolevn

   added

   topic-match-statementPython 3.10's match statementPython 3.10's match statement

   on Sep 7, 2022
4. sobolevn

   self-assigned this

   on Sep 7, 2022
5. AlexWaygood commented on Sep 7, 2022

   

   AlexWaygood

   on Sep 7, 2022

   Last edited by AlexWaygood

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   This is possibly a repro without pattern-matching:

   from typing import Generic, TypeVar
   
   T = TypeVar("T")
   
   class Foo(Generic[T]):
       attr: T
       def __init__(self, attr: T) -> None:
           self.attr = attr
           
   class Bar(Foo[T]): ...
   
   x: Foo[str] = Bar("idk")
   
   if isinstance(x, Bar):
       reveal_type(x.attr)  # Any (should be str!)

   https://mypy-play.net/?mypy=latest&python=3.10&gist=5ba0af842f3a9d75ab0498009e0d92cb
6. sobolevn commented on Sep 7, 2022

   

   sobolevn

   on Sep 7, 2022

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   I think it is something similar to #13607 🤔
   But, I am not sure. I didn't have any time to debug this yet.

   👍React with 👍1Reacted by Alex Waygood
7. sobolevn commented on Sep 8, 2022

   

   sobolevn

   on Sep 8, 2022

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   There are couple of this with this example:

   from typing import Generic, TypeVar
   
   T = TypeVar("T")
   
   
   class Base(Generic[T]):
       ...
   
   class A(Base[T]):
       x: T
   
       __match_args__ = ('x', )
   
       def __init__(self, x: T):
           self.x = x
   
   
   a: Base[str] = A("foo")
   reveal_type(a)  # Base[str] (correct)
   
   match a:
       case A(b):
           reveal_type(b)  # Any (incorrect! Should be builtins.str)

   Right now it is unclear to me: how to properly handle Base in the first place? It does not have x, it does not have __init__, it does not have __match_args__. Should it be even allowed to match A(b)? Probably yes, but this example is hard to process.
8. ariebovenberg commented on Sep 8, 2022

   

   ariebovenberg

   on Sep 8, 2022

   Last edited by ariebovenberg

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   @sobolevn my understanding is that it doesn't matter that Base doesn't have __match_args__ or __init__, this should only matter for the case statements.

   ...but in the end the problem doesn't appear specific to match as @AlexWaygood demonstrates.

   The core problem is that mypy should infer that the T in A[T] can only be str once the value is narrowed to Base[str].
9. sobolevn commented on Sep 8, 2022

   

   sobolevn

   on Sep 8, 2022

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   Yeap, this is another problem :)
   I am actually working on code that @AlexWaygood provided. It is much easier for me to understand.

   🚀React with 🚀1Reacted by Alex Waygood
10. sobolevn commented on Sep 9, 2022

    

    sobolevn

    on Sep 9, 2022

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    Ok, the problem is in this line:

    mypy/mypy/meet.py

    Line 713 in 8147b0c

    return t

    Ideally, we should use something like map_instance_to_supertype to preserve type information about current type arguments.

    But, we need a completely new approach for this. It is not implemented at all.

    So, we take the subtype, go for all the route inside mro tree to find our supertype.
    Then, in reverse order we apply type arguments from one parent to its subclass until we reach the our instance.

    Then, we return the mapped instance and it will solve our problem.
11. sobolevn commented on Sep 9, 2022

    

    sobolevn

    on Sep 9, 2022

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    Ok, I found map_type_from_supertype