def test_args_str_mixed_list():
    """
    args_str should join a list of objects, converting each to a string.
    """
    assert args_str(['a', 1, None]) == 'a 1 None'
def test_args_str_string_list():
    """ args_str should join a list of strings. """
    assert args_str(['a', 'b']) == 'a b'
def test_args_str_string():
    """ args_str should leave string arguments as strings. """
    assert args_str('abc def') == 'abc def'
def test_args_str_none():
    """ args_str should return the string form of non-string arguments. """
    assert args_str(None) == 'None'
def test_args_str_empty_list():
    """ args_str should join return an empty string for an empty list. """
    assert args_str([]) == ''