Non-deterministic test collection when using multiple fixtures

[astrofrog]

The following example causes non-deterministic failures when trying to run tests in parallel:

import pytest

@pytest.fixture(scope="module", params=['A', 'B'])
def fa(request):
    return request.param

@pytest.fixture(scope="module", params=['A', 'B'])
def fb(request):
    return request.param

def test_it(fa, fb):
    pass

$ pytest test.py  -n 4
================================== test session starts ==================================
platform darwin -- Python 3.6.2, pytest-3.1.3, py-1.4.34, pluggy-0.4.0
rootdir: /Users/tom/tmp, inifile:
plugins: cov-2.3.1, mpl-0.8, xdist-1.18.2
gw0 [4] / gw1 [4] / gw2 [4] / gw3 [4]
scheduling tests via LoadScheduling
collecting 0 items / 1 errors
======================================== ERRORS =========================================
_________________________________ ERROR collecting gw3 __________________________________
Different tests were collected between gw1 and gw3. The difference is:
--- gw1

+++ gw3

@@ -1,4 +1,4 @@

 test.py::test_it[A-A]
+test.py::test_it[A-B]
+test.py::test_it[B-B]
 test.py::test_it[B-A]
-test.py::test_it[B-B]
-test.py::test_it[A-B]
================================ 1 error in 1.47 seconds ================================

Note that you need to try multiple times for the failure to appear in some cases. This is using Python 3.6.2 and pytest 3.1.3.

[nicoddemus]

Can you try with pytest-3.2.0? I believe this has been fixed by pytest-dev/pytest#920

[astrofrog]

It does seem to be fixed, thanks!