Add reference docs for pytest.mark.usefixtures

[nicoddemus]

No description provided.

[coveralls]

Coverage increased (+0.05%) to 92.65% when pulling d26a596 on nicoddemus:usefixtures-reference into 7e8f7bf on pytest-dev:master.

[obestwalter]

I wonder if it could be a good idea to add an explicit warning/reminder here that you can only mark test functions like this and not other fixtures to address #1014

[nicoddemus]

Ahh good idea, will do

[obestwalter]

Something like:

"Be aware that although you can use fixtures from oher fixtures by adding them to the function parameters, you can not do this by marking the fixture function with usefixtures."

Maybe even with an example?

This works:

@pytest.fixture
def my_fixture(my_other_fixture):
    [...]

This will not yield the wanted result and currently not result in an error or a warning either.

@pytest.mark.usefixtures("my_other_fixture")
@pytest.fixture
def my_fixture_that_sadly_wont_use_my_other_fixture():
    [...]

I would even suggest to put this in a warning box, because this can be surprising even for experienced pytest users, especially due to the lack of an error atm.

[nicoddemus]

Thanks @obestwalter!

I added the warning but the example I added to the tutorial/normal documentation. IMHO we should keep the reference as lean as possible.

[obestwalter]

perfect :)