Pytest 3.0 does not accept empty list in fixture

[OPpuolitaival]

With pytest 2.x it was very easy to use empty lists in fixture if dynamically run test case did not find anything to test. pytest 3.x seems to accept only None values.

Also when setting empty list in fixture with pytest 3.x the error message is very ugly

example code:

@pytest.fixture(scope="module", ids=list(), params=list())
def temp(request):
   return request.param

def test_temp(temp):
     pass

---

(edited by @The-Compiler to add code blocks)

[The-Compiler]

For reference, this is the behavior with 2.9.2:

============================================ test session starts =============================================
platform linux -- Python 3.5.2, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
rootdir: /home/florian, inifile: 
plugins: mock-1.2
collected 1 items 

x.py s

========================================= 1 skipped in 0.00 seconds ==========================================

With 3.0.0:

============================================ test session starts =============================================
platform linux -- Python 3.5.2, pytest-3.0.0, py-1.4.31, pluggy-0.3.1
rootdir: /home/florian, inifile: 
plugins: hypothesis-2.0.0, catchlog-1.2.1
collected 0 items / 1 errors 

=================================================== ERRORS ===================================================
___________________________________________ ERROR collecting x.py ____________________________________________
.venv/lib/python3.5/site-packages/_pytest/runner.py:163: in __init__
    self.result = func()
.venv/lib/python3.5/site-packages/_pytest/main.py:460: in _memocollect
    return self._memoizedcall('_collected', lambda: list(self.collect()))
.venv/lib/python3.5/site-packages/_pytest/main.py:331: in _memoizedcall
    res = function()
.venv/lib/python3.5/site-packages/_pytest/main.py:460: in <lambda>
    return self._memoizedcall('_collected', lambda: list(self.collect()))
.venv/lib/python3.5/site-packages/_pytest/python.py:404: in collect
    return super(Module, self).collect()
.venv/lib/python3.5/site-packages/_pytest/python.py:318: in collect
    res = self.makeitem(name, obj)
.venv/lib/python3.5/site-packages/_pytest/python.py:330: in makeitem
    collector=self, name=name, obj=obj)
.venv/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py:724: in __call__
    return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
.venv/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py:338: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
.venv/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py:333: in <lambda>
    _MultiCall(methods, kwargs, hook.spec_opts).execute()
.venv/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py:595: in execute
    return _wrapped_call(hook_impl.function(*args), self.execute)
.venv/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py:249: in _wrapped_call
    wrap_controller.send(call_outcome)
.venv/lib/python3.5/site-packages/_pytest/python.py:191: in pytest_pycollect_makeitem
    res = list(collector._genfunctions(name, obj))
.venv/lib/python3.5/site-packages/_pytest/python.py:350: in _genfunctions
    self.ihook.pytest_generate_tests(metafunc=metafunc)
.venv/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py:724: in __call__
    return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
.venv/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py:338: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
.venv/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py:333: in <lambda>
    _MultiCall(methods, kwargs, hook.spec_opts).execute()
.venv/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py:596: in execute
    res = hook_impl.function(*args)
.venv/lib/python3.5/site-packages/_pytest/fixtures.py:1028: in pytest_generate_tests
    ids=fixturedef.ids)
.venv/lib/python3.5/site-packages/_pytest/python.py:846: in parametrize
    ids = idmaker(argnames, argvalues, idfn, ids, self.config)
.venv/lib/python3.5/site-packages/_pytest/python.py:933: in idmaker
    for valindex, valset in enumerate(argvalues)]
.venv/lib/python3.5/site-packages/_pytest/python.py:933: in <listcomp>
    for valindex, valset in enumerate(argvalues)]
.venv/lib/python3.5/site-packages/_pytest/python.py:924: in _idvalset
    if ids is None or ids[idx] is None:
E   IndexError: list index out of range
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
========================================== 1 error in 0.52 seconds ===========================================

[nicoddemus]

Could someone bisect this so we can discuss if this was unintended or there was the intent of making this an error (in which case we should work on the message of course)?

[The-Compiler]

Bisected to be1954a with a git bisect run ./.venv/bin/py.test x.py and 20sec of waiting 😉

cc @palaviv

[nicoddemus]

@The-Compiler thanks! I didn't do it myself because I'm at work right now and would like to get the ball rolling. 😁

[The-Compiler]

@nicoddemus Sorry if that sounded like "why didn't you do it yourself?", it was meant as a "git bisect run is awesome!" 😉

[nicoddemus]

Please, not at all! I was just asking because I couldn't really spend the time doing it myself, thanks for doing it.