Python, QTimer and multiprocessing Pipe

Question

I have two classes: class Bullet and Main Class. I need to the create movement of bullet and I did that, but when bullet moves out of screen I don`t know how to stop function for the moving bullet. Because of that, the program with time gets slower.

import multiprocessing as mp
from PyQt5.QtCore import Qt, QTimer

def bullet(self):
    if self.bull_on[0] is None:
        if self.bull is not None:
            self.timer.stop()
            del self.bull

        self.bull = Bullet(self, self.player.geometry(), self.pipeB1)
        self.bull_on[0] = 1

        if self.player.pix1.position == 'up':
            self.timerCallback = functools.partial(self.bull.move_up, isOn=self.bull_on)
            self.timer.timeout.connect(self.timerCallback)
            self.timer.start(10)

Above code creates instance of class Bullet and start funcion for moving bullet Code below is funcion that starts

def move_up(self, isOn):
    send = [self.rec1.x(), self.rec1.y() - self.counter * 2, 'u']
    self.pipe.send(send)
    val = self.pipe.recv()
    if val > -1:
        self.bullet1 = QPixmap('Images\\Bullet_up.png')
        self.setGeometry(self.rec1.x() + 16, self.rec1.y() - 20 - self.counter*2, self.rec1.width(), self.rec1.height())
        self.setPixmap(self.bullet1)
        self.show()
        self.counter += 1
    else:
        isOn[0] = None
        self.hide()

I use

self.timer.stop()

and

del self.bull

but this does not work. I tried

self.timer.disconnect(self.timerCallback)

but that throws errors saying that pipe is not closed properly.

My question is how to stop the function in the background?