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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def GetUsername(self, event, default_username='-'):
 """Retrieves the username related to the event. Args: event (EventObject):
 event. default_username (Optional[str]):
 default username. Returns: str: username. """

    username = getattr(event, 'username', None)
    if username and username != '-':
      return username

    session_identifier = event.GetSessionIdentifier()
    if session_identifier is None:
      return default_username

    user_sid = getattr(event, 'user_sid', None)
    username = self._knowledge_base.GetUsernameByIdentifier(
        user_sid, session_identifier=session_identifier)
    return username or default_username