Dynamic scope callable for pytest.fixture() cannot be correctly typed

[mtnpke]

When using the pytest.fixture decorator with a callable as the scope argument for dynamic scoping, it is not easily possible to type the callable correctly so that type checkers will, not complain.

Example:

def determine_my_client_scope(_fixture_name: str, config: pytest.Config) -> str:
    return config.getoption("--my-client-fixture-scope", "session")


@pytest.fixture(scope=determine_my_client_scope)
def my_client() -> str:
    return "Hello world!"

gives (using basedpyright):

Argument of type "(_fixture_name: str, config: Config) -> str" cannot be assigned to parameter "scope" of type "_ScopeName | ((str, Config) -> _ScopeName)" in function "fixture"
  Type "(_fixture_name: str, config: Config) -> str" is not assignable to type "_ScopeName | ((str, Config) -> _ScopeName)"
    Type "(_fixture_name: str, config: Config) -> str" is not assignable to type "(str, Config) -> _ScopeName"
      Function return type "str" is incompatible with type "_ScopeName"
        Type "str" is not assignable to type "_ScopeName"
          "str" is not assignable to type "Literal['session']"
[...]

The issue is that the _ScopeName type is private to pytest, so we cannot use it for typing the callable. It would make sense to either make this type public, so people cannot type their functions appropriately, or just use str in the typing.

pytest version: 9.0.2

[The-Compiler]

As a workaround, you should be able to use Literal["session", "package", "module", "class", "function"] which is what _ScopeName is defined as.

But I agree that we should probably make _ScopeName public for typing purposes.