Can someone explain this unexpected raw_input behavior?

[Gabriel Genellina]

En Wed, 23 Jan 2008 18:27:56 -0200, Mike Kent <mrmakent at cox.net> escribió:

> It's often useful for debugging to print something to stderr, and to
> route the error output to a file using '2>filename' on the command
> line.
>
> However, when I try that with a python script, all prompt output from
> raw_input goes to stderr.  Consider the following test program:
> [...]
> This indicates to me that the prompt output of raw_input is being sent
> to stderr.  I did check the source code for raw_input, and it appears
> to be sending it to stdout as expected.

Surely you've seen that in bltinmodule.c, builtin_raw_input calls
PyOS_Readline(PyFile_AsFile(fin), PyFile_AsFile(fout), prompt);
where fin and fout are sys.stdin and sys.stdout respectively.
That function is defined in Parser/myreadline.c, and eventually calls  
PyOS_StdioReadline with the same arguments. But PyOS_StdioReadline doesn't  
use its sys_stdout parameter to output the prompt; instead, it always uses  
stderr, no matter what arguments it receives.
Looking at the svn history, PyOS_StdioReadline always has used stderr; got  
its current signature in  rev 29400 (2002).

Perhaps that behavior is based on the reverse of your use case, which may  
be more common. A script writes most of its output to stdout, and you  
capture that using >filename. If raw_input prompted the user using stdout,  
that would not be visible in this case, so using stderr is more useful.

-- 
Gabriel Genellina