TypeVar Should Accept Arguments When Bound To Generic

[rmorshea]

I've posted about this on gitter but haven't gotten a response. Could someone help me understand how I would construct the type spec for a function def (G[T]) -> G[List[G]] where G is any subclass of MyGeneric class. My current approach results in the commented error:

from typing import TypeVar, Generic

Val = TypeVar("Val")

class MyGeneric(Generic[Val]):
    def __call__(self, a: Val) -> Val: ...

T = TypeVar("T")
G = TypeVar("G", bound=MyGeneric)

def listify_my_generic(g: G[T]) -> G[List[T]]: ...  # error: Type variable "G" used with arguments

Here's a playground link:

https://mypy-play.net/?mypy=0.750&python=3.8&gist=290ef32f0638a648674b98873a263d6f

[rmorshea]

Ok, I've managed to create a work around:

from typing import TypeVar, Generic, List

Val = TypeVar("Val")

class MyGeneric(Generic[Val]):
    def __init__(self, a: Val): ...

T = TypeVar("T")

SingleG = MyGeneric[T]
ListG = MyGeneric[List[T]]

def listify_my_generic(g: SingleG[T]) -> ListG[T]: ...

reveal_type(listify_my_generic(MyGeneric(1)))  # note: Revealed type is 'main.MyGeneric[builtins.list[builtins.int*]]'

However it would be nice if the example above worked - the work around requires more typing and is only able to communicate what's happening so long as descriptive variable names are used.

[JukkaL]

This looks essentially the same as python/typing#548. Discussion can continue there. More generally, this is a type system enhancement and we generally discuss them in the typing issue tracker (or typing-sig@).