Is a mutex really necessary in this piece of code?

The code below shows a class which I use for PyQt threading funcionality. In my main program I instantiate and start this thread (i.e. I use PyQt's moveToThread, and countdown is the run method.). While the thread is running I frequently call the reset method. I'm not sure if the mutex I've implemented is actually necessary, I hope somebody can clear this up for me. I've executed a quick test in which I've commented out the mutex, this gave me no problems, even when I comment out the time.sleep(1) and call the reset method while the thread is running without any delay. However I wan't to be 100% secure, hence the reason for my question here.

import time
from PyQt4.QtCore import *
from threading import  Lock

class Countdown(QObject):
    finished = pyqtSignal()

    def __init__(self, countdownInSec=30):
        super(Countdown, self).__init__()
        self.COUNTDOWN_IN_SEC = countdownInSec
        self._countdownInSec = self.COUNTDOWN_IN_SEC
        self._mutex = Lock()

    @pyqtSlot()
    def countdown(self):
        while self._countdownInSec > 0:
            print(self._countdownInSec)
            self._mutex.acquire()
            try:
                self._countdownInSec -= 1
            finally:
                self._mutex.release()
            time.sleep(1)
        self.finished.emit()

    def increment(self, seconds):
        self._mutex.acquire()
        try:
            self._countdownInSec += seconds
        finally:
            self._mutex.release()

    def reset(self):
        self._mutex.acquire()
        try:
            self._countdownInSec = self.COUNTDOWN_IN_SEC
        finally:
            self._mutex.release()

extraction of main (only the part that is relevant for this question)

    fpIntervUpdCountdownReset = pyqtSignal()                                             

    def __init__(self):
        self.initFlightPathIntervUpdater()

    def initFlightPathIntervUpdater(self):             
       self.fpIntervUpdCountdownThr = QThread()                                         
       self.fpIntervUpdCountdown = countdown.Countdown()
       self.fpIntervUpdCountdown.moveToThread(self.fpIntervUpdCountdownThr)      
       self.fpIntervUpdCountdown.finished.connect(self.fpIntervUpdCountdownThr.quit)    
       self.fpIntervUpdCountdown.finished.connect(self.flightPathIntervUpdate)                  
       self.fpIntervUpdCountdownThr.started.connect(self.fpIntervUpdCountdown.countdown)

   def flightPathIntervUpdateReq(self):                                                 
       if self.fpIntervUpdCountdownThr.isRunning():                                     
           self.fpIntervUpdCountdown.reset()                                            
       else:                                                                            
           print 'start'                                                                
           self.fpIntervUpdCountdownThr.start()                                         

   @pyqtSlot()                                                                          
   def flightPathIntervUpdate(self):                                                    
       print "perform flightPathIntervUpdate"