from pynput.keyboard import Key, Listener

flag = False

def doit():
    while flag:
        print("test")


def released(key):
    global flag
    if key == Key.up:
        flag = True
        doit()
    elif key == Key.down:
        print("stopped")
        flag = False


with Listener(on_release=released) as listener:
    listener.join()