def METHOD_NAME(self, previous, current):
    valid_severities = sorted(StateMachine.Severity, key=StateMachine.Severity.get)
    assert previous in StateMachine.Severity, f"Severity is not one of {', '.join(valid_severities)}"
    assert current in StateMachine.Severity, f"Severity is not one of {', '.join(valid_severities)}"
    if StateMachine.Severity[previous] < StateMachine.Severity[current]:
        return MORE_SEVERE
    elif StateMachine.Severity[previous] > StateMachine.Severity[current]:
        return LESS_SEVERE
    else:
        return NO_CHANGE