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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 mtime(self, key):
 """Return the last modification time for the cache record with key. May be useful for cache instances where the stored values can get 'stale', such as caching file or network resource contents."""

        if key not in self.__dict:
            raise CacheKeyError(key)
        else:
            node = self.__dict[key]
            return node.mtime