Reference counting is also the reason why Python requires a GIL. Incrementing a reference count is an opportunity for a race condition (another thread could read the refcount in between your read and write), which means that every reference count needs to be protected by a lock. Either you do fine-grained locking for each object, or you add a GIL for the whole interpreter. The former will absolutely kill performance (not only do you need to increment a refcount with every assignment or function call, you need to take a lock). The latter makes the whole interpreter thread-hostile.