'Notify that a entity has entered the collection.
Initiator is a token owned by the InstrumentedAttribute that
initiated the membership mutation, and should be left as None
unless you are passing along an initiator value from a chained
operation.'

def fire_append_event(self, item, initiator=None):

    if (initiator is not False):
        if self.invalidated:
            self._warn_invalidated()
        return self.attr.fire_append_event(self.owner_state, self.owner_state.dict, item, initiator)
    else:
        return item
