What does it mean to be notified about "callback errors" by using listener.join() in pynput?

Question

This is a Keylogger and in KeyInput function there is listener.join() but what is that for?

from pynput import keyboard

def KeyInput():
    with keyboard.Listener(on_press=KeyLog) as listener:
        # whenever there is a press on a key on the keyboard, it's directed to KeyLog 
        listener.join()

def KeyLog(key):
    if type(key) == keyboard._win32.KeyCode:
        K = key.char
        # normal characters like letters
    else:
        if 'cmd' in str(key):
            K = str(key).replace('cmd', 'Windows')
        else:
            K = ' '+str(key)+' '
        # keys like ctrl, caps lock, windows, etc
    with open('keylogs.txt', 'a') as File:
        File.write(K+'\n')
        File.close()

KeyInput()
KeyLog()

The documentation of pynput says:

If a callback handler raises an exception, the listener will be stopped. Since callbacks run in a dedicated thread, the exceptions will not automatically be reraised.

To be notified about callback errors, call Thread.join on the listener instance.

What does it mean by "callback error"?

Maybe you should read about [`Thread.join`](https://stackoverflow.com/questions/15085348/what-is-the-use-of-join-in-python-threading#15086113) first. The `listener.join()` is just an frequently-used pattern. More important is the callback defined as `on_press=Keylog`: call this function, when key is pressed. — hc_dev, Jun 25 '22 at 18:51
thanks, from what i get, callback is related to ```on_press=Keylog``` and when that thing raises an exception, ```listener.join()``` stops the exception — MR.PRDX, Jun 25 '22 at 18:58

mkrieger1 · Accepted Answer · 2022-06-25T19:18:45.073

"callback" refers to the KeyLog function being passed as on_press argument to keyboard.Listener.

The listener is a separate thread which calls KeyLog whenever there is a key press event.

If KeyLog raises an exception (e.g. if the "keylogs.txt" file can't be opened), this is a "callback error"; an error in the callback function. But it happens in the listener thread, and the main thread (where KeyInput is called) has no possibility to handle the exception (for example to display a message to the user that the file can't be opened).

In order to be able to handle the exception in the main thread, it must be propagated from the listener thread. This is what listener.join does (which is the join method of the Thread class, since Listener is a subclass):

If there was an exception in the KeyLog function, it will be raised by listener.join() again, so that it can be handled in the main thread.

What does it mean to be notified about "callback errors" by using listener.join() in pynput?

1 Answers1