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.

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.

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.