Can not use autouse fixture to import module - pytest

Question

I have a test module has one autouse fixture
import pytest
#pytest.fixture(autouse=True):
def set_env_config(monkeypatch):
palladium_config = os.path.join(os.path.dirname(os.path.dirname(os.getcwd())), 'config.py')
monkeypatch.setenv('PALLADIUM_CONFIG', palladium_config)
from A import B
and in every followed test within this test module class B is needed, but this importation can not be achieved for any tests.
In the other way, I patch only the environment variable only
#pytest.fixture(autouse=True):
def set_env_config(monkeypatch):
palladium_config = os.path.join(os.path.dirname(os.path.dirname(os.getcwd())), 'config.py')
monkeypatch.setenv('PALLADIUM_CONFIG', palladium_config)
and import the class B in every test case, it succeeded.
Why is that ? why can't I import class within autouse fixture
thanks a lot

Answer 1

I am guessing that you are expecting something like the following:
#pytest.fixture(autouse=True)
def do_an_import():
from A import B
def test_foo():
assert B.do_my_thing() == 'foo'
That doesn't work, while doing the following does what you want:
def test_foo():
from A import B
assert B.do_my_thing() == 'foo'
Unfortunately doing an import in the fixture (the first example) will add B into that fixture function's namespace, but not the test function's namespace.
Similarly, this wouldn't work for the same reason:
#pytest.fixture
def not_an_autouse_fixture():
from A import B
def test_foo(not_an_autouse_fixture):
assert B.do_my_thing() == 'foo'
B is being imported into the fixture's namespace, which is different from the test's namespace. You could instead do:
#pytest.fixture
def Cls():
from A import B
return B
def test_foo(Cls):
assert Cls.do_my_thing() == 'foo'
or you could import it at the top level of your module like:
from A import B
def test_foo(B):
assert B.do_my_thing() == 'foo'