code: def _get_block_header(self, block_hash, num):
          
          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
docs: Get block header by block header hash & number.
  
          :param block_hash:
          :param num:
          :return: