A fixture with autouse set to False will always be used if it overrides a fixture with autouse set to True

[bngo92]

• Include a detailed description of the bug or suggestion
  I took a look at pytest's source code and it looks like pytest records the names of all fixtures with autouse set to True. It then uses this list to determine which fixtures to use, even if the overriding fixture has autouse set to False.
• pytest and operating system versions
  platform linux2 -- Python 2.7.13, pytest-3.4.0, py-1.5.2, pluggy-0.6.0
• Minimal example if possible
  conftest.py

@pytest.fixture(autouse=True)
def foo():
    pass

test_me.py:

@pytest.fixture()
def foo():
    assert False

def test_bar(foo):
    pass

def test_baz():
    pass

Returns the following error:

____________________________________________________________________________ ERROR at setup of test_bar _____________________________________________________________________________

    @pytest.fixture()
    def foo():
>       assert False
E       assert False

____________________________________________________________________________ ERROR at setup of test_baz _____________________________________________________________________________

    @pytest.fixture()
    def foo():
>       assert False
E       assert False

I expect only test_bar to fail. Setting autouse to False for foo in conftest.py correctly causes only test_bar to fail.

[ceridwen]

I've encountered what may be another manifestation of this bug, only with parametrization. I'm doing some dynamic fixture creation based on command-line options. An autouse=True session fixture takes this fixture as an argument:

def pytest_configure(config):
    class DynamicFixturePlugin2(object):
        @pytest.fixture(scope='session', params=['foo, 'bar'] if config.getoption('--foo') else [''])
        def baz(self, request):
            return request.param
    config.pluginmanager.register(DynamicFixturePlugin2(), 'baz-fixture')

I realized that I accidentally created a test that uses that same name as a parameter.

@pytest.mark.parametrize('baz, ['foo', 'bar'])
def test(baz, ...):
   ....

This test just so happened to be the first test in the file it was in. If I then invoke pytest with this file as the first item in a list of test files and that test is the first test in that file, then I get the following error:

ScopeMismatch: You tried to access the 'function' scoped fixture 'baz' with a 'session' scoped request object, involved factories
conftest.py:141:  def autouse_fixture_that_depends_on_dynamic_fixture(request, baz, ...)

If it's not related, tell me and I'll file a separate bug.