def child_state(test_state) -> ChildState:
    """A child state.

    Args:
        test_state: A test state.

    Returns:
        A test child state.
    """
    child_state = test_state.get_substate([ChildState.get_name()])
    assert child_state is not None
    return child_state
A child state.

    Args:
        test_state: A test state.

    Returns:
        A test child state.
    
tests/units/test_state.py