Are Python instance variables thread-safe?

You can use Locks, RLocks, Semaphores, Conditions, Events and Queues.
And this article helped me a lot.
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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