[readmodifywrite]

The GIL is not there to prevent data corruption on shared objects - it only protects the interpreter's internal state. The fact that sometimes you can get away with it is an accident of the GIL's implementation, not a feature anyone should rely on. It also means that you cannot rely on that behavior not to change on a successive versions of CPython.

The only safe way to do shared state between threads in CPython is locks/mutexes/message passing/etc. Even things like a simple addition to an integer are absolutely not made thread safe by the GIL.

[rbanffy]

No, but at least you won't have two cores incrementing the same integer at the same time.