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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 open(self):
 """Opens an existing cache. """

        try:
            self.graph.open(self.cache_uri, create=False)
            self._add_namespaces(self.graph)
            self.is_open = True
        except Exception:
            raise InvalidCacheException('The cache is invalid or not created')