code: def add_block_hash( self, block_hash ):
          
          if len(self.block_hashes) > 2000:
              raise Exception("A getheaders request cannot have over 2000 block hashes")
  
          hash_num = int("0x" + block_hash, 16)
          
          bh = BlockHash()
          bh.block_hash = hash_num
  
          self.block_hashes.append( bh )
          self.hash_stop = hash_num
docs: Append up to 2000 block hashes for which to get headers.