def __init__(self, parent_message, field):
    """Args:
      parent_message: The message whose _Modified() method we should call when
        we receive Modified() messages.
      field: The descriptor of the field being set in the parent message.
    """
    super(_OneofListener, self).__init__(parent_message)
    self._field = field
Args:
      parent_message: The message whose _Modified() method we should call when
        we receive Modified() messages.
      field: The descriptor of the field being set in the parent message.
    
deps/protobuf/python/google/protobuf/internal/python_message.py