Scoped fixtures are not rebuilt when the fixtures they depend on change

[rgilton]

When module-scoped fixture b depends on module-scoped fixture a, I was expecting fixture b to be rebuilt if the value/source of a was changed. In code, I expect all of the following assertions to succeed:

from pytest import fixture

@fixture(scope="module")
def a():
    return 0

@fixture(scope="module")
def b(a):
    return a

def test_b_is_zero(b):
    assert b == 0

class TestClass:
    @fixture(scope="module")
    def a(self):
        return 1

    def test_b_is_one(self, b):
        assert b == 1

However, the fixture that the test within the class is presented with is not as expected:

============================================ test session starts =============================================
platform linux -- Python 3.14.0, pytest-9.0.2, pluggy-1.6.0 -- /home/rob/tmp/fixtures/.venv/bin/python3
cachedir: .pytest_cache
rootdir: /home/rob/tmp/fixtures
configfile: pyproject.toml
collected 2 items                                                                                            

test_a.py::test_b_is_zero PASSED                                                                       [ 50%]
test_a.py::TestClass::test_b_is_one FAILED                                                             [100%]

================================================== FAILURES ==================================================
__________________________________________ TestClass.test_b_is_one ___________________________________________

self = <test_a.TestClass object at 0x7f235325fb10>, b = 0

    def test_b_is_one(self, b):
>       assert b == 1
E       assert 0 == 1

test_a.py:24: AssertionError
========================================== short test summary info ===========================================
FAILED test_a.py::TestClass::test_b_is_one - assert 0 == 1
======================================== 1 failed, 1 passed in 0.03s =========================================

% uv pip list
Package   Version
--------- -------
iniconfig 2.3.0
packaging 25.0
pluggy    1.6.0
pygments  2.19.2
pytest    9.0.2

[nileshsingal]

I think this is expected behavior based on how pytest scopes and caches fixtures.

Since b is module-scoped, it’s created once for the module and then reused. The first
test that requests b resolves a using the module-level fixture and that value of
b is cached for the remainder of the module.

Overriding a at the class level doesn’t cause pytest to invalidate or rebuild b,
because that would break the “one instance per scope” guarantee for module-scoped
fixtures.

It might be worth clarifying this in the docs, as it’s easy to assume that overriding
a dependency would automatically cause dependent fixtures to be rebuilt.