Star operator gets confused when the type has multiple generics #11138

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1. KotlinIsland

   added

   bugmypy got something wrongmypy got something wrong

   on Sep 18, 2021
2. sobolevn commented on Sep 18, 2021

   

   sobolevn

   on Sep 18, 2021

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   @KotlinIsland this line does not seem to be correct: b: list[list[int]] = [*a]. That's exactly what error: List item 0 has incompatible type "A[int, Iterator[int]]"; expected "List[int]" tells you about.
   Here b should be list[int]:

   from typing import Iterator, Generic, TypeVar
   T = TypeVar("T")
   T2 = TypeVar("T2")
   
   class A(Iterator[T2], Generic[T, T2]):
       ...
   a: A[int, Iterator[int]] = A()
   reveal_type((*a,))
   reveal_type([*a])
   b: list[int] = [*a]

   It works: https://mypy-play.net/?mypy=latest&python=3.10&gist=e9ba2aada4295a07d24c5508eadfb5f0
   Similar to reveal_type((*a,)) which reveals Revealed type is "builtins.tuple[builtins.int*]"
3. KotlinIsland commented on Sep 19, 2021

   

   KotlinIsland

   on Sep 19, 2021

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   @sobolevn actually it should be a list[Iterator[int]]
4. KotlinIsland commented on Sep 19, 2021

   

   KotlinIsland

   on Sep 19, 2021

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   I think this is clearer: https://mypy-play.net/?mypy=latest&python=3.10&gist=e7a3eb908190fd06c04032459aae0d70
5. KotlinIsland commented on Sep 19, 2021

   

   KotlinIsland

   on Sep 19, 2021

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   from typing import Generic, TypeVar
   T = TypeVar("T")
   T2 = TypeVar("T2")
   
   class A(Iterator[T2], Generic[T, T2]):
       ...
       
   a: A[str, int]
   reveal_type((*a,))  # note: Revealed type is "builtins.tuple[builtins.str*]"
   b: list[int] = [*a]  # error: List item 0 has incompatible type "A[str, int]"; expected "int"
6. JelleZijlstra commented on Sep 19, 2021

   

   JelleZijlstra

   on Sep 19, 2021

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   @KotlinIsland's latest gist does look pretty buggy. For (*a,) it infers tuple[str, ...], which is wrong because a is an Iterator[Iterator[int]] instead.
7. JelleZijlstra commented on Sep 19, 2021

   

   JelleZijlstra

   on Sep 19, 2021

   Last edited by JelleZijlstra

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   I like this example of the bug:

   from typing import Generic, Iterator, TypeVar
   T = TypeVar("T")
   U = TypeVar("U")
   
   class X(Iterator[U], Generic[T, U]):
       pass
   
   x: X[str, int]
   reveal_type(list(x))  # N: Revealed type is "builtins.list[builtins.int*]"
   reveal_type([*x])  # N: Revealed type is "builtins.list[builtins.str*]"
8. sobolevn commented on Sep 20, 2021

   

   sobolevn

   on Sep 20, 2021

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   Oh, I see. Will try to fix it tomorrow 👍

   🎉React with 🎉2Reacted by Jelle Zijlstra and KotlinIsland
9. sobolevn commented on Sep 20, 2021

   

   sobolevn

   on Sep 20, 2021

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   Yeap, solved using map_instance_to_supertype(actual_type, Iterable) here:

   mypy/mypy/argmap.py

   Lines 168 to 173 in b3ff2a6

   	if actual_type.type.fullname == 'builtins.list':
   	# List *arg.
   	return actual_type.args[0]
   	elif actual_type.args:
   	# TODO: Try to map type arguments to Iterable
   	return actual_type.args[0]

   But, I need changes from #11151 to make it work correctly. Right now it is blocked.
   @JelleZijlstra I would appreciate your review to get it merged! 🙏
10. JelleZijlstra commented on Sep 20, 2021

    

    JelleZijlstra

    on Sep 20, 2021

    Last edited by JelleZijlstra

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    Nice catch! I figured we were assuming somewhere that the first generic arg was for Iterable, but couldn't find the right place in the code.

    🚀React with 🚀1Reacted by sobolevn
11. hauntsaninja

    closed this as completedin #11151on Oct 31, 2021