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

Short answer: no.

Long answer: generally not.

While CPython's GIL makes single opcodes thread-safe, this is no general behaviour. You may not assume that even simple operations like an addition is a atomic instruction. The addition may only be half done when another thread runs.

And as soon as your functions access a variable in more than one opcode, your thread safety is gone. You can generate thread safety, if you wrap your function bodies in locks. But be aware that locks may be computationally costly and may generate deadlocks.