import sys, readline

sys.stdout.write('foo ')
raw_input()

When trying the above on Debian, 2.6.6 using gnome-terminal, typing a character then hitting backspace deletes "foo " as well.

I'm not sure if this is a bug or the expected behavior when writing to stdout directly rather than passing the string to raw_input() (for my particular use case that's not an option though).

One possible workaround seems to be to delete the trailing space from write() and move it to raw_input:

sys.stdout.write('foo')
raw_input(' ')

Then backspace seems to work properly. This has something to do with readline because when it's not imported, it also works as expected (but other things break obviously).

Maybe you need to call sys.stdin.flush() before raw_input?

In any way, 2.6 is in security mode, so we need to reproduce this with current versions: 2.7, 3.2 or 3.3.

Reproduced on 2.7.

(flushing stdin/out doesn't help)

Reproduced on 3.3 head. Looking at the documentation of the C readline
library, it needs to know the length of the prompt in order to display
properly, so this seems to be an acknowledged limitation of the underlying
library rather than a bug on our side.

Still, this behavior is surprising and undesirable. I would suggest adding
a note to the docs for the readline module, directing users to write:

    input("foo> ")

instead of:

    sys.stdout.write("foo> ")
    input()

You're right, as this little C program verifies:

#include <stdio.h>
#include <stdlib.h>
#include <readline/readline.h>

int main() {
   printf("foo ");
   char* buf = readline("");
   free(buf);

   return 0;
}

Passing ' ' seems to be a suitable workaround for those who can't pass the text directly to raw_input though (such is the case where you have special classes who handle output).

> Still, this behavior is surprising and undesirable. I would suggest
> adding a note to the docs for the readline module
+1.

Hi,
submitting a patch for the documentation.
Thanks

The patch looks find for Python 3. The sample code should probably be adapted to raw_input() for Python 2.

Actually, there should either be a space before the double-colons, or the full stops should be removed. So either of these options:

. . . when a backspace is typed. ::
. . . when a backspace is typed::

Also, I'd change the patch to use a note directive.

I wonder if this information would be better off under the input() function, rather than under the Readline module itself.

Also see Issue 17337, about control codes in the Readline prompt. Since these issues both relate to the prompt, it might be worth documenting both in the same section or paragraph.

I agree with Martin that this belongs in the input() rather than readline() docs. However, the input() does is quite concise, and the sole example specifies the right way to use it:

https://docs.python.org/3/library/functions.html#input

I find it hard to justify real estate in this doc in which to explain what happens if you combine input with readline and do the wrong thing. 

I suggest we leave it as it is.