def __init__(self, parent_message):
    """Args:
      parent_message: The message whose _Modified() method we should call when
        we receive Modified() messages.
    """
    # This listener establishes a back reference from a child (contained) object
    # to its parent (containing) object.  We make this a weak reference to avoid
    # creating cyclic garbage when the client finishes with the 'parent' object
    # in the tree.
    if isinstance(parent_message, weakref.ProxyType):
      self._parent_message_weakref = parent_message
    else:
      self._parent_message_weakref = weakref.proxy(parent_message)

    # As an optimization, we also indicate directly on the listener whether
    # or not the parent message is dirty.  This way we can avoid traversing
    # up the tree in the common case.
    self.dirty = False
Args:
      parent_message: The message whose _Modified() method we should call when
        we receive Modified() messages.
    
deps/protobuf/python/google/protobuf/internal/python_message.py