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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_snapshot(self, snapshot_id):
 """ Return the snapshot or raise a 404 if the snapshot is unknown """

        try:
            return self._snapshots[snapshot_id]
        except KeyError:
            raise aiohttp.web.HTTPNotFound(text="Snapshot ID {} doesn't exist".format(snapshot_id))