Title: raw_input behavior incorrect if readline not enabled
Type: behavior Stage: test needed
Components: Interpreter Core Versions: Python 3.4, Python 2.7
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: Daniel.Gonzalez, bhuvan, eric.araujo, ezio.melotti, ggenellina, martin.panter, mdomingues, taleinat
Priority: normal Keywords: patch

Created on 2008-01-24 20:21 by skip.montanaro, last changed 2015-11-22 01:13 by martin.panter.

File name Uploaded Description Edit
promptOutputFix.patch mdomingues, 2012-09-24 01:21 A proposed patch to print raw_input prompts to standard out. review
promptOutputFix3.patch mdomingues, 2012-09-24 01:26 Fixes the input prompt issue on the Python3 branch. review
Messages (11)
msg61652 - (view) Author: Skip Montanaro (skip.montanaro) * Date: 2008-01-24 20:21
From a thread on python-dev...

Mike Kent mike.kent at
Thu Jan 24 16:33:47 CET 2008

Recently I was trying to debug an old python program who's maintenance I
inherited.  I was using the quick-and-dirty method of putting some 'print
>>sys.stderr' statements in the code, and then running the command with
'2>filename' appended to the end of the command line.  Imagine my surprise
to see that all of the prompt text from the program's raw_input calls were
also disappearing from the screen output, and appearing in the stderr
output routed to the file.

The latest documentation for raw_input states "If the prompt argument is
present, it is written to standard output without a trailing newline."
I posted a question regarding the observed behavior to comp.lang.python
and Gabriel Genellina (thanks Gabriel!) pointed out that despite the
documentation, raw_input was hard-coded to always output its prompt text
to stderr.

This raises two questions:
1. Shouldn't the current documentation be corrected to state that raw_input
writes its prompt to standard error?
2. Is this really the hard-coded behavior we want?  I don't think my
use-case is that odd; in fact, what I find very odd is that the prompt
output is send to stderr.  I mean, I'm printing the prompt for a question,
not some error message. Can there not at least be an optional parameter to
indicate that you want the output sent to stdout rather than stderr?

... after a few responses ...

Guido van Rossum guido at
Thu Jan 24 21:09:12 CET 2008

On Jan 24, 2008 11:41 AM, Mike Kent <mike.kent at> wrote:
> Interesting point about whether GNU readline is installed.  My setup
is RedHat
> Linux, with Python 2.5 that I built and installed myself.  GNU
readline is not,
> in fact, installed.  If you look at Python2.5/Parser/myreadline.c,
> PyOS_StdioReadline, line 125, you will see that prompt output is being
sent to
> stderr.  As best as my Python-fu can determine, this is the code used
to output
> a raw_input prompt (thanks again to Gabriel Genellina for pointing me
in the
> right direction.)
> It's entirely likely that the difference in what I am seeing and what
you guys
> are seeing is caused by my not having GNU readline installed. 
> the behavior without it seems wrong, and is certainly different from the
> documentation.

msg61655 - (view) Author: Gabriel Genellina (ggenellina) Date: 2008-01-24 22:31
GNU readline is configured as to prompt the user using standard output, 
and read input from standard input; if this is the desired behavior it 
would be easy to provide a simple patch so input/raw_input behave that 
way even when readline is not used.
msg116936 - (view) Author: Mark Lawrence (BreamoreBoy) * Date: 2010-09-20 13:53
Any *NIX gurus who can sort this one?
msg154049 - (view) Author: Éric Araujo (eric.araujo) * (Python committer) Date: 2012-02-23 04:22
From reading the code for raw_input in 2.7 or input in 3.3 (Python/bltinmodule.c:1573), it looks to me that stdout is used, which would mean this issue is fixed.  However I browsed the file history and could not find the commit that changed this, and my C skills are limited, so I’m adding Ezio to nosy to have another pair of eyes confirm.
msg171087 - (view) Author: Michael Domingues (mdomingues) Date: 2012-09-24 01:21
The code that dictates this behavior is in /Parser/myreadline.c and has not been rectified yet in either Python 2.7 ( or the default branch ( Specifically, within these functions, references to standard error should actually be references to standard out.

The attached file is a proposed patch for this bug on the 2.7 branch, bringing interpreter behavior into accordance with the Python documentation (, which states that the prompt is written to standard out, as opposed to standard error.
msg171088 - (view) Author: Michael Domingues (mdomingues) Date: 2012-09-24 01:26
Also uploading a patch for the Python3.2 branch.
msg177966 - (view) Author: Daniel Gonzalez (Daniel.Gonzalez) Date: 2012-12-23 09:33
Please see this stackoverflow thread where more information is given about this issue:
msg223493 - (view) Author: Martin Panter (martin.panter) * (Python committer) Date: 2014-07-20 01:35
I experimented with various redirections to /dev/null, files, and other terminal windows on Linux. Current behaviour I am seeing seems to be something like this:

* Prefers prompting to stderr if both stdout and stderr are terminals
* Prefers prompting to stdout if neither are terminals
* Prompts to the non-terminal if only one of stderr and stdout is a terminal. Surely this one should be the other way around if there is going to be a preference at all?
msg241693 - (view) Author: Bhuvan Arumugam (bhuvan) Date: 2015-04-21 02:26
For the record, this bug is still open. 

The proposed patch is not merged in any of branches.

The prompt for raw_input in all versions, go to stderr.
msg246392 - (view) Author: Tal Einat (taleinat) * (Python committer) Date: 2015-07-07 07:11
See also issue #24402: input() uses sys.__stdout__ instead of sys.stdout for prompt
msg255080 - (view) Author: Martin Panter (martin.panter) * (Python committer) Date: 2015-11-22 01:13
The input() implementation is a bit like this:

def input(prompt):
    if stdin and stdout are the original file descriptors, and are terminals:
        return PyOS_Readline(sys.stdin, sys.stdout, prompt)
        sys.stdout.write(prompt)  # Writes to stdout
        return sys.stdin.readline()

def PyOS_StdioReadline(stdin, stdout, prompt):
    '''Default implementation of PyOS_Readline()'''
    sys.stderr.write(prompt)  # Writes to stderr
    return stdin.readline()

def call_readline(stdin, stdout, prompt):
    '''Implementation of PyOS_Readline() in the "readline" module'''
    rl_instream = stdin
    rl_outstream = stdout  # Readline writes to stdout
    return readline_until_enter_or_signal(prompt)

It looks like PyOS_StdioReadline() has always written to stderr. The stdin and stdout parameters of PyOS_Readline() were added later, in revision dc4a0336a2a3.

I think the changes to myreadline.c will also affect the interactive interpreter prompt. But we have Issue 12869 open to change that to stdout, so maybe the change is okay.

Since input() should no longer depend on any instance of stderr, perhaps the check for “lost sys.stderr” should also be removed.

It may be worth applying any changes in myreadline.c to the independent version in pgenmain.c as well, just for consistency.
Date User Action Args
2015-11-22 01:13:22martin.pantersetmessages: + msg255080
2015-11-21 23:09:28martin.panterlinkissue25692 superseder
2015-07-07 07:11:05taleinatsetnosy: + taleinat
messages: + msg246392
2015-04-21 02:26:54bhuvansetnosy: + bhuvan
messages: + msg241693
2014-07-20 01:35:18martin.pantersetnosy: + martin.panter

messages: + msg223493
versions: + Python 3.4
2014-02-03 19:04:58BreamoreBoysetnosy: - BreamoreBoy
2012-12-23 09:33:18Daniel.Gonzalezsetnosy: + Daniel.Gonzalez
messages: + msg177966
2012-09-24 01:26:28mdominguessetfiles: + promptOutputFix3.patch

messages: + msg171088
2012-09-24 01:21:31mdominguessetfiles: + promptOutputFix.patch

nosy: + mdomingues
messages: + msg171087

keywords: + patch
2012-02-23 04:22:36eric.araujosetnosy: + ezio.melotti, eric.araujo
messages: + msg154049
2010-09-20 13:53:20BreamoreBoysetnosy: + BreamoreBoy

messages: + msg116936
versions: + Python 2.7, - Python 2.6
2010-05-20 20:36:08skip.montanarosetnosy: - skip.montanaro
2010-01-29 04:42:52brian.curtinsetstage: test needed
versions: - Python 2.5
2008-01-24 22:32:00ggenellinasetnosy: + ggenellina
messages: + msg61655
2008-01-24 20:33:44christian.heimessetpriority: normal
2008-01-24 20:21:28skip.montanarosettype: behavior
2008-01-24 20:21:17skip.montanarocreate