python windows mouse hook crash

Question

I'm setting up a mouse hook in python like that:

def listen():
    global hook_id

    def low_level_handler(aCode, wParam, lParam):
        if aCode != win32con.HC_ACTION:
            return ctypes.windll.user32.CallNextHookEx(hook_id, aCode, wParam, lParam)

        return ctypes.windll.user32.CallNextHookEx(hook_id, aCode, wParam, lParam)

    # Our low level handler signature.
    CMPFUNC = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_int, ctypes.c_int, ctypes.POINTER(ctypes.c_void_p))
    # Convert the Python handler into C pointer.
    pointer = CMPFUNC(low_level_handler)


    # Hook both key up and key down events for common keys (non-system).
    hook_id = ctypes.windll.user32.SetWindowsHookExA(win32con.WH_MOUSE_LL, pointer,
                                             GetModuleHandle(None), 0)
    # Register to remove the hook when the interpreter exits. Unfortunately a
    # try/finally block doesn't seem to work here.
    atexit.register(ctypes.windll.user32.UnhookWindowsHookEx, hook_id)

def process_msg():
    while True:
        status, msg = PeekMessage(None, 0, 0, win32con.PM_REMOVE)
        if status == 0:
            break
        TranslateMessage(ctypes.byref(msg))
        DispatchMessage(ctypes.byref(msg))

process_msg is then later called in a loop

Everything seems to be working fine until I do SendInput that simulates a mouse click from within the same app. Once I simulate click, there's a crash. What could possibly be the cause?

Thanks.

Answer 1

It looks like def low_level_handler was going out of scope and being garbage collected (?) / removed from memory. After I moved it out of def listen, it's all working.