That example is not thread-safe _behavior-wise_ under the GIL. The block is composed of several individual operations, and the interpreter can preempt a thread

That example is not thread-safe _behavior-wise_ under the GIL. The block is composed of several individual operations, and the interpreter can preempt a thread between any one of them. On the other hand, it is safe from a memory-access perspective; the read from `self._next_id` will never dereference a partially-mutated invalid pointer or read a partially-mutated value.