Wrong type hint after unpacking/swapping #9731

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1. NiklasRosenstein

   added

   bugmypy got something wrongmypy got something wrong

   on Nov 18, 2020
2. NiklasRosenstein

   changed the title [-]Wrong type hint after unpacking[/-] [+]Wrong type hint after unpacking/swapping[/+] on Nov 18, 2020
3. NiklasRosenstein commented on Nov 18, 2020

   

   NiklasRosenstein

   on Nov 18, 2020

   Author

   More actions

   Splitting the swap/unpacking into two lines works:

   import typing as t
   
   def spam(val: t.Union[t.Type, None] = None) -> None:
   
     def eggs(type_: t.Type) -> None:
       pass
   
     reveal_type(val)
     if isinstance(val, type):
       reveal_type(val)
       foo = val
       val = None
       reveal_type(val)
       reveal_type(foo)
       return eggs(foo)

   $ mypy test.py 
   test.py:9: note: Revealed type is 'Union[Type[Any], None]'
   test.py:11: note: Revealed type is 'Type[Any]'
   test.py:14: note: Revealed type is 'None'
   test.py:15: note: Revealed type is 'Type[Any]'
   
4. JelleZijlstra commented on Nov 18, 2020

   

   JelleZijlstra

   on Nov 18, 2020

   Member

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   Sounds like we update the inferred local type of val too soon. Hopefully this shouldn't be hard to fix.
5. hauntsaninja

   added

   priority-1-normal

   on Nov 19, 2020
6. NiklasRosenstein commented on Jun 4, 2021

   

   NiklasRosenstein

   on Jun 4, 2021

   Last edited by NiklasRosenstein

   Author

   More actions

   I just hit this again in a different scenario.

   from dataclasses import dataclass
   
   class A:
     pass
   
   @dataclass
   class B(A):
     a: A
     v: int
   
   x: A = B(A(), 1)
   
   v: int = 0
   if isinstance(x, B):
     x, v = x.a, x.v  # error: "A" has no attribute "v"
   
   print(x, v)