Can a pytest test change the value of a session fixture and affect subsequent tests?

Question

Let's say that a pytest session fixture returns a dictionary and this dictionary is then manipulated within a test. Would this affect subsequent tests which use the same fixture?

Answer 1

Yes. And that is trivial to verify. Exactly one of these tests will fail:

import pytest

@pytest.fixture(scope="session")
def d():
    return {}

def test_one(d):
    assert not d
    d["k"] = "v"

def test_two(d):
    assert not d
    d["k"] = "v"

If the fixture scope is changed to "function" (the default scope) then both tests will pass, since the dict will be created anew each time.