code: def serialize_number(x, fmt=SER_BINARY, outlen=None):
      
      ret = b''
      if fmt == SER_BINARY:
          while x:
              x, r = divmod(x, 256)
              ret = six.int2byte(int(r)) + ret
          if outlen is not None:
              assert len(ret) <= outlen
              ret = ret.rjust(outlen, b'\0')
          return ret
      assert fmt == SER_COMPACT
      while x:
          x, r = divmod(x, len(COMPACT_DIGITS))
          ret = COMPACT_DIGITS[r:r + 1] + ret
      if outlen is not None:
          assert len(ret) <= outlen
          ret = ret.rjust(outlen, COMPACT_DIGITS[0:1])
      return ret
docs: Serializes `x' to a string of length `outlen' in format `fmt'