code: def _from_bytes(bytes):
          
          assert len(bytes) >= 4
          # calculate checksum
          s = (-sum(bytes)) & 0x0FF
          bin = array('B', bytes + [s])
          return ':' + asstr(hexlify(array_tobytes(bin))).upper()
docs: Takes a list of bytes, computes the checksum, and outputs the entire
          record as a string. bytes should be the hex record without the colon
          or final checksum.
  
          @param  bytes   list of byte values so far to pack into record.
          @return         String representation of one HEX record