How to parameterize a TypeVar by another TypeVar

[eike-fokken]

Hi!

I have the following Protocol:

from typing import Protocol, TypeVar

ValueType = TypeVar("ValueType")


# Protocol for types, that provide a [] operator for reading and writing.
class IndexibleProtocol(Protocol[ValueType]):
    def __setitem__(self, key: int, value: ValueType) -> None:
        ...

    def __getitem__(self, key: int) -> ValueType:
        ...
Indexible = TypeVar("Indexible", bound=IndexibleProtocol)

I can now use this for type annotations like so:

def get_some_value(container: Indexible) -> Any:
    return container[0]

But if I try

def get_some_value(container: Indexible) -> ValueType:
    return container[0]

I get a mypy error telling me "A function returning TypeVar should receive at least one argument containing the same TypeVar.

Can I somehow index the Indexible type by ValueType, so that I could (hopefully, I don't understand it super well) write something like this:

def get_some_value(container: Indexible[ValueType]) -> ValueType:
    return container[0]

[sobolevn]

It is not possible right now. See:

• Higher-Kinded TypeVars #548
• https://returns.readthedocs.io/en/latest/pages/hkt.html

[dmoisset]

At least in that example, you don't need the second typevar

You could write:

def get_some_value(container: IndexibleProtocol[ValueType]) -> ValueType:
    return container[0]

[eike-fokken]

Thanks, this is also what I came up with in the mean time. My working (or so it seems up to now) code is this:

from typing import Generic, Protocol, TypeVar

ValueType = TypeVar("ValueType")

# Protocol for types, that provide a [] operator for reading and writing.
class Indexible(Generic[ValueType], Protocol):
    def __setitem__(self, key: int, value: ValueType) -> None:
        ...

    def __getitem__(self, key: int) -> ValueType:
        ...

which can then be used for type annotations as you show:

def get_some_value(container: Indexible[ValueType]) -> ValueType:
    return container[0]