With pytest 3:

=============================== warnings summary ===============================
test.py:66
  test.py:66: RemovedInPytest4Warning: Applying marks directly to parameters is deprecated, please use pytest.param(..., marks=...) instead.
  For more details, see: https://docs.pytest.org/en/latest/parametrize.html
    @pytest.mark.parametrize ('file_name, encoding', gen_test_params())
  test.py:66: RemovedInPytest4Warning: Applying marks directly to parameters is deprecated, please use pytest.param(..., marks=...) instead.
  For more details, see: https://docs.pytest.org/en/latest/parametrize.html
    @pytest.mark.parametrize ('file_name, encoding', gen_test_params())
  test.py:66: RemovedInPytest4Warning: Applying marks directly to parameters is deprecated, please use pytest.param(..., marks=...) instead.
  For more details, see: https://docs.pytest.org/en/latest/parametrize.html
    @pytest.mark.parametrize ('file_name, encoding', gen_test_params())
  test.py:66: RemovedInPytest4Warning: Applying marks directly to parameters is deprecated, please use pytest.param(..., marks=...) instead.
  For more details, see: https://docs.pytest.org/en/latest/parametrize.html
    @pytest.mark.parametrize ('file_name, encoding', gen_test_params())
-- Docs: https://docs.pytest.org/en/latest/warnings.html
============== 359 passed, 4 xfailed, 4 warnings in 47.63 seconds ==============

With pytest 4:

==================================== ERRORS ====================================
___________________________ ERROR collecting test.py ___________________________
/usr/lib/python2.7/site-packages/pluggy/hooks.py:284: in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
/usr/lib/python2.7/site-packages/pluggy/manager.py:68: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
/usr/lib/python2.7/site-packages/pluggy/manager.py:62: in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
/usr/lib/python2.7/site-packages/_pytest/python.py:226: in pytest_pycollect_makeitem
    res = list(collector._genfunctions(name, obj))
/usr/lib/python2.7/site-packages/_pytest/python.py:403: in _genfunctions
    self.ihook.pytest_generate_tests(metafunc=metafunc)
/usr/lib/python2.7/site-packages/pluggy/hooks.py:284: in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
/usr/lib/python2.7/site-packages/pluggy/manager.py:68: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
/usr/lib/python2.7/site-packages/pluggy/manager.py:62: in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
/usr/lib/python2.7/site-packages/_pytest/python.py:133: in pytest_generate_tests
    metafunc.parametrize(*marker.args, **marker.kwargs)
/usr/lib/python2.7/site-packages/_pytest/python.py:995: in parametrize
    function_definition=self.definition,
/usr/lib/python2.7/site-packages/_pytest/mark/structures.py:114: in _for_parametrize
    if len(param.values) != len(argnames):
E   TypeError: object of type 'MarkDecorator' has no len()
!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!
=========================== 1 error in 0.36 seconds ============================