OS: Mac 10.7.2
Python: 2.7.1

Setting a pre input hook in readline has no effect.  This simple example illustrates the problem:

#!/usr/bin/python

import readline

def hook():
        readline.insert_text(' from pre_input_hook')
        readline.redisplay()

readline.set_pre_input_hook(hook)

while True:
        line = raw_input('Prompt ("stop" to quit): ')
        if line == 'stop':
                break
        print 'ENTERED: "%s"' % line


The hook never gets called, thus "from pre_input_hook" is never displayed.

Since you mention 2.7.1, presumably you are using the Apple-suppled Python 2.7.1 in OS X.  That Python is not linked with GNU readline, rather the BSD libedit library.  As a workaround you could try installing the third-party readline package from PyPI which replaces the Python redline module with one that is statically linked with GNU readline (see  http://pypi.python.org/pypi/readline).  On the other hand, if there are more differences in behavior between Python readline linked with GNU readline and with BSD libedit, they should be documented.

1) Issue is still present with libedit on OSX 10.8

2) Libedit on OSX doesn't call rl_pre_input_hook when using the '#if defined(HAVE_RL_CALLBACK) && defined(HAVE_SELECT)' branch in the readline extension (that is, not calling readline(3), but using rl_callback_read_char).

3) When I patch pyconfig.h to enable the code patch that does use readline the callback gets called, but that doesn't affect the readline buffer.

Manually ensuring that the pre_input_hook is called at the right time is easy enough, but I don't have time to research why readline.insert_text (and/or readline.redisplay) doesn't work. The source code at <http://www.opensource.apple.com/source/libedit/libedit-31/src/readline.c> looks fine.

Surely this should be fixed in Editline, not Python?

The problem with fixing in editline is that Apple ships a copy of editline and that is used when build CPython by default.

IMHO it would be acceptable to work around this in the readline extension if that can be done in a clean way.