`python-config` outputs a usage instruction message in case either the `--help` option or invalid arguments are provided.

However there is a small different in the output I/O between the python and shell script versions of `python-config`. In the python version, the message always prints to `stderr`.

https://github.com/python/cpython/blob/bc1c49fa94b2abf70e6937373bf1e6b5378035c5/Misc/python-config.in#L15-L18

def exit_with_usage(code=1):
    print("Usage: {0} [{1}]".format(
        sys.argv[0], '|'.join('--'+opt for opt in valid_opts)), file=sys.stderr)
    sys.exit(code)

while in the shell script version it always prints to `stdout`.

https://github.com/python/cpython/blob/bc1c49fa94b2abf70e6937373bf1e6b5378035c5/Misc/python-config.sh.in#L5-L9

exit_with_usage ()
{
    echo "Usage: $0 --prefix|--exec-prefix|--includes|--libs|--cflags|--ldflags|--extension-suffix|--help|--abiflags|--configdir|--embed"
    exit $1
}

This inconsistency does not affect most users of `python-config`, who runs the script interactively. However it might cause issues when run programmatically.

I think python-config is modelled after pkg-config: what does the latter do?

Also, I’m not sure it would be appropriate to change behaviour in existing releases.

Thanks. I didn't know about the `pkg-config` thing. I just checked with pkg-config on macOS

`pkg-config --help` prints usage instructions to stdout

pkg-config followed by invalid options prints output to stderr, for example `pkg-config --asdf` prints `Unknown option --asdf` to stderr.

The PR I submitted does something similar, making both python and shell script versions of `python-config` to print out the usage instruction message to stdout when --help is provided and stderr when given invalid arguments.