def test_invalid_arg_produces_message(self):
    out = self.assertRunOK('badfilename')
    self.assertRegex(out, b"Can't list 'badfilename'")
def test_invalid_arg_produces_message(self):
    out = self.assertRunOK('badfilename')
    self.assertRegex(out, b"Can't list 'badfilename'")