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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 listeners_iter(self):
 """Return an iterator over the mapping of event => listeners bound. The listener list(s) returned should **not** be mutated. NOTE(harlowja):
 Each listener in the yielded (event, listeners) tuple is an instance of the :py:class:`~.Listener` type, which itself wraps a provided callback (and its details filter callback, if any). """

        topics = set(six.iterkeys(self._topics))
        while topics:
            event_type = topics.pop()
            try:
                yield event_type, self._topics[event_type]
            except KeyError:
                pass