code: def _get_block_header_by_hash(db: BaseDB, block_hash: Hash32) -> BlockHeader:
          
          validate_word(block_hash, title="Block Hash")
          try:
              header_rlp = db[block_hash]
          except KeyError:
              raise HeaderNotFound("No header with hash {0} found".format(
                  encode_hex(block_hash)))
          return _decode_block_header(header_rlp)
docs: Returns the requested block header as specified by block hash.
  
          Raises BlockNotFound if it is not present in the db.