How to type yield fixtures

[OlgaPaw]

Hello!

I have an example pytest fixture that uses yield

@pytest.fixture()
def foo() -> str:
    yield "foo"

Mypy gives an error The return type of a generator function should be "Generator" or one of its supertypes<

How can I type that fixture, so mypy understands it?

[The-Compiler]

I'd expect Iterator[str], see https://mypy.readthedocs.io/en/stable/kinds_of_types.html#generators

[bluetech]

Iterator[str] is actually not strong enough; the correct incantation is

from typing import Generator

@pytest.fixture()
def foo() -> Generator[str, None, None]:
    yield "foo"

I'm hoping one day generics could have defaults, in which case the type would be just Generator[str], but for now the full thing is needed.

[graingert]

fyi you can parametrize type aliases:

from typing import Generator, TypeVar
import pytest

T = TypeVar("T")

YieldFixture = Generator[T, None, None]


@pytest.fixture
def foo() -> YieldFixture[str]:
    yield "foo"

Live demo: https://mypy-play.net/?mypy=latest&python=3.8&flags=strict