Type checkers report the type of TestReport.when as Union[builtins.str, None] (coming from the BaseReport base class) rather than the correct type for TestReport of Literal["setup", "call", "teardown"].

(I checked this with mypy 1.10.0 and pyright 1.1.342)

This looks to be due to the fact that the BaseReport annotation is on a class attribute, which takes precedence over the instance attribute assigned within the derived TestReport class.

Repro

def pytest_runtest_logreport(report: TestReport) -> None:
    reveal_type(report.when)

mypy 1.10.0 reports:

note: Revealed type is "Union[builtins.str, None]"

(The actual type is Literal["setup", "call", "teardown"])

Suggested Solution

I think this can be resolved by adding a class-level type annotation to TestReport with the narrower type (the same applies to location):

@final
class TestReport(BaseReport):
    # [...]
    when: Literal["setup", "call", "teardown"]
    location: tuple[str, int | None, str]
    # [...]