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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_block_header(self, block_hash, num):
 """Get block header by block header hash & number. :param block_hash: :param num: :return: """

        header_key = header_prefix + num + block_hash

        block_header_data = self.db.get(header_key)
        header = rlp.decode(block_header_data, sedes=BlockHeader)
        return header