[i80and]

It's not atomic even with the GIL, though: another thread can run in between the bytecode's load and increment, right?

The GIL's guarantees didn't extend to this.

[Groxx]

There is NB_INPLACE_ADD... but I'm struggling to find enough details to be truly confident :\ possibly its existence is misleading other people (thus me) to think += is a single operation in bytecode.

Or, on further reading, maybe it applies to anything that implements `_iadd_` in C. Which does not appear to include native longs: https://github.com/python/cpython/blob/main/Objects/longobje...