Why does python -V write to the error stream?

Question

I was writing a script to inspect python's version on my system and I've noticed that python -V writes to the error stream, while python -h, for instance, uses the standard output. Is there a good reason for this behavior?

Answer 1

-h used to print to stderr too as you see here from main.c

usage(int exitcode, char* program)
{
fprintf(stderr, usage_line, program);
fprintf(stderr, usage_top);
fprintf(stderr, usage_mid);
fprintf(stderr, usage_bot, DELIM, DELIM, PYTHONHOMEHELP);
exit(exitcode);
/*NOTREACHED*/
}

...

if (help)
 usage(0, argv[0]);

if (version) {
 fprintf(stderr, "Python %s\n", PY_VERSION);
 exit(0);

The current main.c has changed the way usage is defined

usage(int exitcode, char* program)
{
FILE *f = exitcode ? stderr : stdout;

fprintf(f, usage_line, program);
if (exitcode)
 fprintf(f, "Try `python -h' for more information.\n");
else {
 fputs(usage_1, f);
 fputs(usage_2, f);
 fputs(usage_3, f);
 fprintf(f, usage_4, DELIM);
 fprintf(f, usage_5, DELIM, PYTHONHOMEHELP);
}

So usage uses stdout for -h and stderr for -Q.

I can't see any evidence of a good reason one way of the other. Possibly it cannot be changed now without breaking backward compatibility

Answer 2

Probably for no good reason, some digging revealed the the patch adding the options, but I can find any references to why the different streams are used in the discussion about the patch.

Answer 3

Why?

Because it's not the actual output of your actual script.

That's the long-standing, standard, common, typical, ordinary use for standard error: everything NOT output from your script.

Answer 4

The -h option also used to print to stderr because it is not part of the output of your program, i.e. the output is not produced by your Python script but by the Python interpreter itself.

As for why they changed the -h to use stdout? Try typing python -h with your terminal window set to the standard 24 lines. It scrolls off the screen.

Now most people would react by trying python -h |less but that only works if you send the output of -h to the stdout instead of stderr. So there was a good reason for making -h go to stdout, but no good reason for changing -V.