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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 to_protobuf(self):
 """Return a protobuf corresponding to the key. :rtype: :class:`.entity_pb2.Key` :returns: The protobuf representing the key. """

        key = _entity_pb2.Key()
        key.partition_id.project_id = self.project

        if self.namespace:
            key.partition_id.namespace_id = self.namespace

        for item in self.path:
            element = key.path.add()
            if "kind" in item:
                element.kind = item["kind"]
            if "id" in item:
                element.id = item["id"]
            if "name" in item:
                element.name = item["name"]

        return key