Showing how to edit events from Quartz in Python

alter_keys.py

#! /usr/bin/python2.6
# I COULD ONLY GET IT TO RUN IN PYTHON2.6
# Running it in python2.7, installed from homebrew results in a segfault.
# I haven't been able to investigate why.
# Code translated from http://osxbook.com/book/bonus/chapter2/alterkeys/
# License: http://ljos.mit-license.org/
from Quartz import (
    CGEventGetIntegerValueField, CGEventSetIntegerValueField,
    kCGKeyboardEventKeycode,
    kCGEventKeyDown, kCGEventKeyUp,
    CGEventTapCreate,
    kCGSessionEventTap, kCGHeadInsertEventTap,
    CFMachPortCreateRunLoopSource,
    kCFAllocatorDefault,
    CFRunLoopGetCurrent,
    kCFRunLoopCommonModes,
    CFRunLoopAddSource,
    CGEventTapEnable,
    CFRunLoopRun)

# This callback will be invoked every a key is pressed.
def eventCallBack(proxy, etype, event, refcon):
    # The incoming keycode.
    keycode = CGEventGetIntegerValueField(event, kCGKeyboardEventKeycode)

    #Swap 'a' and 'z'
    if keycode == 0:
        keycode = 6
    elif keycode == 6:
        keycode = 0

    CGEventSetIntegerValueField(event, kCGKeyboardEventKeycode, keycode)
    return event

if __name__ == '__main__':
    eventMask = (1 << kCGEventKeyDown) | (1 << kCGEventKeyUp)
    eventTap = CGEventTapCreate(kCGSessionEventTap,
                                kCGHeadInsertEventTap,
                                0,
                                eventMask,
                                eventCallBack,
                                None);
    if not eventTap:
        print "failed to create event tap\n"
    else:
        runLoopSource = CFMachPortCreateRunLoopSource(kCFAllocatorDefault,
                                                      eventTap,
                                                      0)
        CFRunLoopAddSource(CFRunLoopGetCurrent(),
                           runLoopSource,
                           kCFRunLoopCommonModes);
        CGEventTapEnable(eventTap, True);
        CFRunLoopRun();