Rule to disallow assertions in fixtures

[m-burst]

Rule request

Description

The rule should check that there are no assert statements in fixtures.

Rationale

• assertions in fixtures are a sign of bad test design, as the test logic becomes splattered between setup/teardown code and actual test code
• inspired by eslint-plugin-jasmine rule no-expect-in-setup-teardown

[sobolevn]

I don't agree on this one. Many people use fixtures for custom complex asserts. I can even say that this is the best practice.

Examples:

• https://github.com/wemake-services/wemake-python-styleguide/blob/master/tests/test_visitors/conftest.py#L12-L37
• https://github.com/wemake-services/wemake-python-styleguide/blob/master/tests/test_visitors/conftest.py#L40-L66

[m-burst]

@sobolevn These cases are not what I meant. If the fixture function returns a function which has some assertions in it, that is fine, since the function is called during test code execution and thus becomes part of the test code.

What I see as a violation is making assertions in the fixture function itself, i.e. during setup/teardown:

@pytest.fixture()
def my_thing():
    thing = get_my_thing()
    assert thing.some_attribute
    return thing

In the above case, if the assertion on thing needs to be made, it should be a separate test case.

[sobolevn]

Yes, this seems legit! 👍