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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 get_topic(self):
 """ Returns the considered topic if applicable. """

        pk = self.kwargs.get(self.topic_pk_url_kwarg, None)
        if not pk:
            return
        if not hasattr(self, '_topic'):
            self._topic = get_object_or_404(Topic, pk=pk)
        return self._topic