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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: [on topic] Re: readline trick needed
Date: Tue, 16 Oct 2012 10:30:01 +0200
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Steven D'Aprano wrote:

> I'm working with the readline module, and I'm trying to set a key
> combination to process the current command line by calling a known
> function, *and* enter the command line.
> 
> Something along the lines of:
> 
> * execute function spam() in some context where it can access
>   the current command line as a string
> * enter the command line
> 
> Function spam() may or may not modify the command line.

> (P.S. I'm aware of IPython, I want to get this working in the standard
> CPython interpreter.)

If IPython does what you want why don't you have a look at the source?

Anyway, here's what I came up with (no warranties as it was all trial-and-
error):

$ cat readline_callback.py
import ctypes
import ctypes.util
from ctypes import c_int, c_char_p
import readline
import sys

rlname = ctypes.util.find_library("readline")

# int rl_add_defun (const char *name, rl_command_func_t *function, int key)
# typedef int rl_command_func_t (int, int);
# int rl_done
# char * rl_line_buffer

rl = ctypes.CDLL(rlname)

RL_COMMAND_FUNC = ctypes.CFUNCTYPE(c_int, c_int, c_int)

intercepted_line = None
def lovely_spam(a, b):
    global intercepted_line
    print
    intercepted_line = c_char_p.in_dll(rl, "rl_line_buffer").value
    rl.rl_insert_text("*" + intercepted_line)
    c_int.in_dll(rl, "rl_done").value = 1
    return 0

def control(c):
    return ord(c.upper())-64

rl.rl_add_defun("lovely-spam", RL_COMMAND_FUNC(lovely_spam), control("P"))

#rl.rl_add_defun("lovely-spam", RL_COMMAND_FUNC(lovely_spam), -1)
#readline.parse_and_bind("Control-P: lovely-spam")

$ python -i readline_callback.py 
>>> alpha = 42
>>> alpha
1764

The relevant documentation:
http://cnswww.cns.cwru.edu/php/chet/readline/readline.html
http://docs.python.org/library/ctypes.html