How can I use python xlib to generate a single keypress?

I'll use ctypes to show you how it could work, but porting it to python-xlib should be straightforward. So lets start with loading the library:

import ctypes
X11 = ctypes.CDLL("libX11.so")

and defining the structures needed:

class Display(ctypes.Structure):
    """ opaque struct """

class XKeyEvent(ctypes.Structure):
    _fields_ = [
            ('type', ctypes.c_int),
            ('serial', ctypes.c_ulong),
            ('send_event', ctypes.c_int),
            ('display', ctypes.POINTER(Display)),
            ('window', ctypes.c_ulong),
            ('root', ctypes.c_ulong),
            ('subwindow', ctypes.c_ulong),
            ('time', ctypes.c_ulong),
            ('x', ctypes.c_int),
            ('y', ctypes.c_int),
            ('x_root', ctypes.c_int),
            ('y_root', ctypes.c_int),
            ('state', ctypes.c_uint),
            ('keycode', ctypes.c_uint),
            ('same_screen', ctypes.c_int),
        ]

class XEvent(ctypes.Union):
    _fields_ = [
            ('type', ctypes.c_int),
            ('xkey', XKeyEvent),
            ('pad', ctypes.c_long*24),
        ]

X11.XOpenDisplay.restype = ctypes.POINTER(Display)

Now we just need to send the event to the root window:

display = X11.XOpenDisplay(None)
key = XEvent(type=2).xkey #KeyPress
key.keycode = X11.XKeysymToKeycode(display, 0xffcc) #F15
key.window = key.root = X11.XDefaultRootWindow(display)
X11.XSendEvent(display, key.window, True, 1, ctypes.byref(key))
X11.XCloseDisplay(display)

This minimal example worked well for me (just using F2 instead). The same can be done to send a KeyRelease event. If a special window is to be targeted, key.window should be set appropriately.

I'm not sure, if it's necessary to use the XEvent union, since it'd worked with the XKeyEvent all alone for me, but it's better to be safe.