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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_key_alias_from_cache(self, key_arn):
        '''
        Find a key's alias by looking up its key_arn in the KEY_METADATA
        cache. This function will only work after a key has been lookedup by
        its alias and is meant as a convenience function for turning an ARN
        that's already been looked up back into its alias.
        '''
        for alias in self.KEY_METADATA:
            if self.KEY_METADATA[alias]['KeyMetadata']['Arn'] == key_arn:
                return alias
        return None