def assert_called_once(mock, expected_args=None, expected_kwargs=None):
    """Assert ``mock`` called once with expected args/kwargs.

    :param mock.Mock mock: The Mock to assert against.
    :param expected_args:
        A tuple containing the expected positional arguments.
    :param expected_kwargs:
        A dictionary containing the expected keyword arguments.
    """
    assert_true(isinstance(mock, Mock))
    if expected_args is None:
        expected_args = ()
    if expected_kwargs is None:
        expected_kwargs = {}
    assert_equal(1, mock.call_count)
    args, kwargs = mock.call_args
    assert_equal(expected_args, args)
    assert_equal(expected_kwargs, kwargs)