Are Python instance variables thread-safe?

Question

OK, check following codes first:

class DemoClass():

    def __init__(self):
        #### I really want to know if self.Counter is thread-safe. 
        self.Counter = 0

    def Increase(self):
        self.Counter = self.Counter + 1

    def Decrease(self):
        self.Counter = self.Counter - 1

    def DoThis(self):
        while True:
            Do something

            if A happens:
                self.Increase()
            else:
                self.Decrease()

            time.sleep(randomSecs)

    def DoThat(self):
        while True:
            Do other things

            if B happens:
                self.Increase()
            else:
                self.Decrease()

            time.sleep(randomSecs)

    def ThreadSafeOrNot(self):
        InterestingThreadA = threading.Thread(target = self.DoThis, args = ())
        InterestingThreadA.start()

        InterestingThreadB = threading.Thread(target = self.DoThat, args = ())
        InterestingThreadB.start()

I'm facing same situation as above. I really want to know if it's thread-safe for self.Counter, well if not, what options do I have? I can only think of threading.RLock() to lock this resource, any better idea?

Answer 1

Variables accessed by multiple threads at the same time will lead to race conditions and unwanted behaviours. You must rely on Locks, RLocks, Semaphores, Conditions, Events or Queues.

The following article helps to understand the difference between each and when to use them in a multi-threaded environment: