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?

Solution 1

You can use Locks, RLocks, Semaphores, Conditions, Events and Queues.
And this article helped me a lot.
Check it out: Laurent Luce's Blog

Solution 2

Using the instance field self.Counter is thread safe or "atomic". Reading it or assigning a single value - even when it needs 4 bytes in memory, you will never get a half-changed value. But the operation self.Counter = self.Counter + 1 is not because it reads the value and then writes it - another thread could change the value of the field after it has been read and before it is written back.

So you need to protect the whole operation with a lock.

Since method body is basically the whole operation, you can use a decorator to do this. See this answer for an example: https://stackoverflow.com/a/490090/34088

Solution 3

No, it is not thread safe - the two threads are essentially modifying the same variable simultaneously. And yes, the solution is one of the locking mechanisms in the threading module.

BTW, self.Counter is an instance variable, not a class variable.

Solution 4

self.Counter is an instance variable, so each thread has a copy.

If you declare the variable outside of __init__(), it will be a class variable. All instances of the class will share that instance.

Solution 5

The Atomos library provides atomic (thread-safe) wrappers for Python primitives and objects, including atomic counters. It uses single-writer/multiple-reader locks.

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Comments

[Eli Bendersky]

Out of curiosity, what do you mean by "the instance field ... is thread safe"?

[Shane]

@EliBendersky: I guess he means operations like self.Counter = Value are thread-safe. Check this article I just found: effbot.org/pyfaq/…

[Harry Johnston]

Surely multiple threads can access the same instance of an object?

[Austin Henley]

@HarryJohnston Be very careful with that. It can add quite a bit of complexity. You may have to do some form of locking.

[Harry Johnston]

@AustinHenley: yes, that's the point. The OP wanted to know whether instance variables were thread-safe or not.

[Basel Shishani]

Assigning a value is not thread-safe. Say you have self.Counter=1 followed by self.Counter=2 then x=self.Counter. Then x can get 1 or 2 depending on thread switching, which makes assigning a value not thread-safe.

[Aaron Digulla]

What I said is that a single assignment is atomic (except for 64-bit types). See stackoverflow.com/questions/4756536/…

[variable]

How is increment a problem as the instance variable of class exists only within one thread. Is an instance variable shared across threads?

[variable]

I think that only class variable is not thread safe. Instance variables should be thread safe unless the class instance is created globally, and passed to the thread. What do you think?

[variable]

I think that only class variable is not thread safe. Instance variables should be thread safe unless the class instance is created globally, and passed to the thread. What do you think?

[Aaron Digulla]

@variable In the code in the question, the same instance is shared between threads.

[Maxime de Pachtere]

@variable because instances can be passed to different thread, your point dosn't make sense. Things are or aren't thread safe regardless of their implementation.

[Orhan G. Hafif]

This is the information that I was looking for actually. People who don't know C, don't even think about this while mutexing every operation. Still, I'll put an uppercase comment line with this info but I'll remove mutexes from my code to improve performance.