Let the pytest fixtures use a custom user factory

[jefftriplett]

Closes #125, open since 2020.

The gap

TestCase subclasses can set a user_factory attribute and make_user() will build users through it instead of User.objects.create_user(). The tp and tp_api fixtures have no class of their own to set it on, so pytest users could never reach that hook. If your User model has required fields or no username field, tp.make_user() simply could not build one.

The option

[tool.pytest.ini_options]
test_plus_user_factory = "myapp.factories.UserFactory"

A dotted path to the factory. Unset, which is the default, nothing changes: make_user() calls User.objects.create_user() exactly as before. Bad values raise pytest.UsageError naming the problem rather than leaking an ImportError or AttributeError out of a fixture.

The part that was not obvious

Assigning t.user_factory = factory on the fixture instance looks right and does nothing. make_user() is a classmethod and reads cls.user_factory, so an instance attribute never reaches it. The fixtures build a small subclass instead:

cls = TestCase if factory is None else type("TestCase", (TestCase,), {"user_factory": factory})

My first attempt did the instance assignment. The option loaded correctly, tp.user_factory was set, and users were still created by create_user(). The end to end test is what caught it, which is why it is written the way it is rather than asserting on the attribute alone.

Backwards compatibility

Purely additive. The option defaults to unset, TestCase.user_factory still defaults to None, and the 105 pre-existing tests are untouched and passing.

Tests

Nine added, covering the loader and the real behaviour:

• unset and empty string both resolve to None
• a valid dotted path resolves to the factory object
• a path with no module, an unimportable module, and a missing attribute each raise pytest.UsageError with a useful message
• tp.user_factory is None by default, and make_user() still works without a factory
• end to end: pytest runs in a subprocess with -o test_plus_user_factory=... and asserts the created user's email comes from the factory's sequence, which is the field that proves the factory ran

114 passed, 1 skipped, 2 xfailed, up from 105. Lint clean, docs build clean.

Docs

docs/usage.md gains a "A custom user factory" section under pytest usage, showing both the pyproject.toml and pytest.ini forms and saying when you would want it.