This seems like a request for extra feature(s), rather than a bug report.

FWIW I agree with Victor’s hesitation in <https://bugs.python.org/issue6135#msg123024>. I doubt it is worth adding special support to have multiple pipes that all use text mode (universal_newlines=True) but with different encoding settings. If you really want this, just add an external TextIOWrapper, or string encoding:

process = Popen(command, stdin=PIPE, stdout=PIPE, ...)

# Manually wrap with TextIOWrapper. Probably risks deadlocking in the general case, due to buffering, so this is a bad idea IMO.
input_writer = io.TextIOWrapper(process.stdin, "iso-8859-1")
output_reader = io.TextIOWrapper(process.stdout, "utf-8")

# Better: use communicate(), or a custom select() loop or similar:
input_string = input_string.encode("iso-8859-1")
[output_string, _] = process.communicate(input_string)
output_string = output_string.decode("utf-8")

Also I’m not enthusiastic about adding encoding etc parameters for a single PIPE scenario. What is wrong with leaving it in binary mode in the Popen call, and encoding, decoding, or using TextIOWrapper as a separate step?