1   The problem

This problem has been submitted by Paul Winkler, and the text below has been reformatted from our email exchanges.

Let's say I have a module, call it manglers.py, containing this simple python function:

def break_on_whitespace(some_string):
    words = some_string.split()
    return '\n'.join(words)

The goal is telling Emacs about this function so that I can call it on a region of text and replace the region with the result of the call. And bind this action to a key, of course, let's say [f7].

The Emacs buffer ought to be handled in some way. If this is not on the Emacs Lisp side, it has to be on the Python side, but we cannot escape handling the buffer. So, there is an equilibrium in the work to do for the user, that could be displaced towards Emacs Lisp or towards Python.

2   Python side

Here is a first draft for the Python side of the problem:

from Pymacs import lisp

def break_on_whitespace():
    start = lisp.point()
    end = lisp.mark(True)
    if start > end:
        start, end = end, start
    text = lisp.buffer_substring(start, end)
    words = text.split()
    replacement = '\n'.join(words)
    lisp.delete_region(start, end)
    lisp.insert(replacement)

interactions = {break_on_whitespace: ''}

For various stylistic reasons, this could be rewritten into:

from Pymacs import lisp
interactions = {}

def break_on_whitespace():
    start, end = lisp.point(), lisp.mark(True)
    words = lisp.buffer_substring(start, end).split()
    lisp.delete_region(start, end)
    lisp.insert('\n'.join(words))

interactions[break_on_whitespace] = ''

The above relies, in particular, on the fact that for those Emacs Lisp functions used here, start and end may be given in any order.

3   Emacs side

On the Emacs side, one would do:

(pymacs-load "manglers")
(global-set-key [f7] 'manglers-break-on-whitespace)