reduce overloads using TypeVarTuple and Unpack

[michaeloliverx]

Summary

Reduce overloads for get and aget and allow an unlimited number of service types to be returned.

Uses PEP 646 – Variadic Generics which means typing-extensions is required for python versions below 3.11.

Opened as a draft as mypy doesn't support TypeVarTuple and Unpack yet but support has been merged python/mypy#16354.

pyright has supported them for a while now.

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[michaeloliverx]

@hynek opened as a draft but what do you think?

[hynek]

Wow if it worked reliably, that would be absolutely amazing.

The question is how breaking it would be to say that this feature only works with Mypy 1.7(?) 🤔

[michaeloliverx]

The question is how breaking it would be to say that this feature only works with Mypy 1.7(?) 🤔

Hard question to answer I am not sure, you can actually enable some support in the current version with:

--enable-incomplete-feature=TypeVarTuple --enable-incomplete-feature=Unpack

Wow if it worked reliably, that would be absolutely amazing.

I think I jumped the gun a bit, its not working as I expected 😢, there doesn't seem to be a way to express (type[T]) -> T in the 2nd overload:

from typing import TypeVar, TypeVarTuple, Unpack, assert_type, overload

T = TypeVar("T")
Ts = TypeVarTuple("Ts")


@overload
def get(svc_type: type[T], /) -> T:
    ...


@overload
def get(*svc_types: Unpack[Ts]) -> tuple[Unpack[Ts]]:
    ...


def get(*svc_types: type) -> object:
    """
    Get services of *svc_types*.

    Instantiate them if necessary and register their cleanup.

    Returns:
            ``svc_types[0]`` | ``tuple[*svc_types]``: If one service is
            requested, it's returned directly. If multiple are requested, a
            tuple of services is returned.
    """
    ...


a = get(int)
assert_type(a, int)

a, b, c = get(int, str, bool)
assert_type(a, int)
assert_type(b, str)
assert_type(c, bool)

The return type is the same of what was passed i.e not an instance

example.py:36: error: Expression is of type "type[str]", not "str"  [assert-type]
example.py:37: error: Expression is of type "type[bool]", not "bool"  [assert-type]

Close but not correct, maybe I will ask in the mypy issue tracker if its possible to express such a type.

[hynek]

Aw, good luck!

[michaeloliverx]

Yes unfortunately it is a limit of the type system, python/mypy#16394.

Maybe some day.

[hynek]

Thanks for trying tho; that would've been awesome!