Correctly support DB access in parallel Tox testing

[adamantike]

Hi!

Recently, I have migrated some Django projects to run their tests using Tox. Because of the number of tests and Tox environments, it was configured to run tests in parallel. This required a similar approach as what's currently handled by pytest-django when running tests with pytest-xdist: Database renaming to avoid collisions.

A simple fixture like django_db_modify_db_settings_xdist_suffix, to add a suffix to database names, was enough to fix database collisions, so I wonder if pytest-django should also handle Tox testing in these scenarios. And, now more than ever, with Tox implementing real support for parallel execution.

I have the time to work on this, and provide what I've implemented so far, if this request is accepted!

[blueyed]

Yes, it makes sense to use something like django_db_modify_db_settings_tox_suffix for this.
But maybe there could be a single *_suffix then?
It should also handle xdist in parallel tox then.

[adamantike]

I think it depends on how Pytest's fixture resolution order works. If it's deterministic (by design, and this contract won't change), I think that separate fixtures make sense. If not, a single django_db_modify_db_settings_parallel_suffix could be enough to avoid different DB names with each run.

[blueyed]

I agree that a single fixture is better in general (for overriding/extending).
I think we should have django_db_modify_db_settings_parallel_suffix that just uses django_db_modify_db_settings_xdist_suffix and django_db_modify_db_settings_tox_suffix by default, doing nothing.

Both would append a suffix by themselves, i.e. xdist in tox would work.

[adamantike]

That's where this comes into play:

If it's deterministic (by design, and this contract won't change), I think that separate fixtures make sense.

By having this implementation, is the execution order determined by the position they have as arguments?

@pytest.fixture(scope="session")
def django_db_modify_db_settings_parallel_suffix(
    django_db_modify_db_settings_tox_suffix,
    django_db_modify_db_settings_xdist_suffix,
):
    skip_if_no_django()

I can investigate how it's evaluated in pytest codebase otherwise.

As a sidenote, I can see the django_db_modify_db_settings and django_db_modify_db_settings_xdist_suffix fixtures are not being exposed in fixtures.__all__.