Is the += operator thread-safe in Python?

Question

I want to create a non-thread-safe chunk of code for experimentation, and those are the functions that 2 threads are going to call.

c = 0

def increment():
          


        

Answer 1

(note: you would need global c in each function to make your code work.)

Is this code thread safe?

No. Only a single bytecode instruction is ‘atomic’ in CPython, and a += may not result in a single opcode, even when the values involved are simple integers:

>>> c= 0
>>> def inc():
...     global c
...     c+= 1

>>> import dis
>>> dis.dis(inc)

  3           0 LOAD_GLOBAL              0 (c)
              3 LOAD_CONST               1 (1)
              6 INPLACE_ADD         
              7 STORE_GLOBAL             0 (c)
             10 LOAD_CONST               0 (None)
             13 RETURN_VALUE        

So one thread could get to index 6 with c and 1 loaded, give up the GIL and let another thread in, which executes an inc and sleeps, returning the GIL to the first thread, which now has the wrong value.

In any case, what's atomic is an implementation detail which you shouldn't rely on. Bytecodes may change in future versions of CPython, and the results will be totally different in other implementations of Python that do not rely on a GIL. If you need thread safety, you need a locking mechanism.