Allow applying generic types to TypeVarTuple

[eltoder]

Feature

If Ts is a TypeVarTuple, using it in a generic type Foo will be equivalent to applying Foo to every component of the tuple. I.e. Foo[Ts] will be logically equivalent to (Foo[T1], Foo[T2], ...) where T1, T2 are the components of the tuple. Foo[Ts] itself will be treated like a TypeVarTuple, except that its components are not plain type variables, but an application of Foo.

Pitch

This will help giving better types to zip-like functions. Currently, these functions are typed using @overload, for example:

class map(Iterator[_S]):
    @overload
    def __new__(cls, func: Callable[[_T1], _S], iter1: Iterable[_T1], /) -> Self: ...
    @overload
    def __new__(cls, func: Callable[[_T1, _T2], _S], iter1: Iterable[_T1], iter2: Iterable[_T2], /) -> Self: ...
    @overload
    def __new__(
        cls, func: Callable[[_T1, _T2, _T3], _S], iter1: Iterable[_T1], iter2: Iterable[_T2], iter3: Iterable[_T3], /
    ) -> Self: ...
    ...

class zip(Iterator[_T_co]):
    @overload
    def __new__(cls, iter1: Iterable[_T1], /, *, strict: bool = ...) -> zip[tuple[_T1]]: ...
    @overload
    def __new__(cls, iter1: Iterable[_T1], iter2: Iterable[_T2], /, *, strict: bool = ...) -> zip[tuple[_T1, _T2]]: ...
    @overload
    def __new__(
        cls, iter1: Iterable[_T1], iter2: Iterable[_T2], iter3: Iterable[_T3], /, *, strict: bool = ...
    ) -> zip[tuple[_T1, _T2, _T3]]: ...
    ...

This is very verbose and error-prone, and also introduces an arbitrary limit for the number of arguments. (typeshed stubs support up to 6 arguments for map and zip and up to 10 for itertools.product.)

With the proposed feature these can be typed simply as:

class map(Iterator[_S]):
    def __new__(cls, func: Callable[[*_Ts], _S], *iters: *Iterable[_Ts]) -> Self: ...

class zip(Iterator[_T_co]):
    def __new__(cls, *iters: *Iterable[_Ts], *, strict: bool = ...) -> zip[tuple[*Ts]]: ...

[erictraut]

A feature like this would likely require a PEP and an update to the Python typing spec. The Python typing forum would be a good place to discuss the idea.

A related idea was proposed here if you're interested in commenting (or collaborating) on that proposal.

[eltoder]

@erictraut Thank you. Indeed this is a very similar proposal, and I agree with your comments in that thread.

[jorenham]

See also python/typing#1216