type hinting for request.param (such as SubRequest[MyType])

[altendky]

I could presumably submit a PR for this if it making SubRequest a Generic is deemed a good approach.

What's the problem this feature will solve?

When type hinting a parameter for a request fixture the request.param attribute is typed as Any and I have to # type: ignore[no-any-return].

Describe the solution you'd like

Long term, a plugin would be nice to automatically relate the .param to it's source in the @pytest.fixture() decoration. Short term, Request could just be a Generic.

This would allow me to use request: _pytest.fixtures.SubRequest[MyType] instead of the type ignore and get any useful hints on the .param attribute inside the fixture as well as return type verification.

Alternative Solutions

I have not tried anything beyond the type ignore.

Additional context

None.

[asottile]

I believe you should utilize cast(whatevertype, request.param) ?

[altendky]

Not sure how I missed this response. Thank you and sorry.

Is that a suggestion as a better thing than the ignore or as a better thing than making SubRequest a Generic? It seems to me that SubRequest[whatevertype] is both more concise and closer on the path to automatic handling than cast(). But yes, cast() is a better option than ignore as I had mentioned.

[bluetech]

SubRequest is currently not public, but it should be exposed eventually. I agree it makes sense to make it generic by the param type -- currently it is Any. Some difficulties to just doing it:

• I don't fully understand SubRequest, so I'm afraid of exposing it myself (FixtureRequest -- SubRequest's supertype -- is exposed, but doesn't have param). Some effort is needed to untangle what is really going on there.

• When the fixture is not parametrized, request.param is currently simply not assigned, i.e. accessing it gives AttributeError. No way to represent "undefined" in mypy that I know of. We'd need to make it work somehow, or assign a special value. Needs care because I'm sure there are users/plugins which rely on it.

• Python typing currently does not support defaults for type variables in generics, this means if SubRequest becomes generic, every user needs to specify SubRequest[XXX] even though most uses of the request fixture are not parametrized, so it's an ergonomic hit. Possibly this can be worked around by asking non-parameterized users to use FixtureRequest instead, of by adding a type alias with the default (this requires inventing another name).

[altendky]

Always something more going on... thanks for the explanation.

[danon]

@bluetech SubRequest also doesn't have param, at least not always.

@fixture()
def my_fixture(request: SubRequest):
    type(request) == SubRequest # true
    request.param() # params is missing, it simply isn't defined,

Probably due to this condition.

Looks like LSP broken to me.