"Signature of a method incompatible with supertype" vs pytest fixtures

[serhiy-storchaka]

I don't know whether it is idiomatic or not, but I use the following approach to parametrize pytest tests. Tests are defined as method in a class. Parameters are defined as fixtures in a class. Fixtures are overridden in subclasses to run tests with different parameters.

For example:

import pytest


class TestA:
    @pytest.fixture
    def foo(self) -> int:
        return 42

    def test_foo(self, foo: int) -> None:
        assert foo > 0


@pytest.fixture
def bar() -> str:
    return "abc"


class TestB(TestA):
    @pytest.fixture
    def foo(self, bar: str) -> int:  # line 20
        return len(bar)

TestA.test_foo and TestB.test_foo are executed with different values of foo. The problem is that fixtures can depend on other fixtures, and signatures for methods defining fixture can be different. MyPy complains:

t.py:20: error: Signature of "foo" incompatible with supertype "TestA"
t.py:20: note:      Superclass:
t.py:20: note:          def foo(self) -> int
t.py:20: note:      Subclass:
t.py:20: note:          def foo(self, bar: str) -> int

It is not real error in the user code that the methods have different signatures. These methods are not called directly. They are called by pytest which provides all arguments.

I can add "# type: ignore", but I do not want to lose static type checking completely. It is useful to check that the type of the parameter matches the returning type of corresponding fixture function.

mypy 0.961 (compiled: yes)
pytest 7.1.2
Python 3.9.13

[The-Compiler]

This doesn't seem like something pytest can do anything about. Maybe it should be a discussion rather than an issue?

I've seen this message being disputed for other reasons too:

• Why is "incompatible with supertype" an error? python/mypy#1237

Note that mypy supports adding error codes to type ignores.

[serhiy-storchaka]

At least partially it can be solved from pytest side. Changing the return type of pytest.fixture from Union[FixtureFunctionMarker, FixtureFunction] to Union[FixtureFunctionMarker, Callable[..., Any]] silences MyPy. Of course it is at the cost of losing some information, but is this information used anywhere?

[RonnyPfannschmidt]

@nicoddemus @The-Compiler i wonder if we should finally remove FixtureFunction for a FixtureDefinition (which is not callable)

the deprecation errors since a while now and its about timme we take it away - that way fixture definitions will no longer be mistaken as method definitions

[nicoddemus]

Deprecation errors or warnings you mean?

[RonnyPfannschmidt]

@nicoddemus we hard fail with a pytest.fail these days

[nicoddemus]

Ahh then sure, I'm 👍