Don't forget that sys.setcheckinterval() is more of a hint than a requirement. It's easy to wrap sys.setcheckinterval () in a try/except or an if-hasattr, and just ignore the case where it doesn't exist ('it won't be necessary'). Doing the same thing with a 'critical section' would be a lot harder. I would also assume a 'critical section' should not allow threads in extension modules, even if they explicitly allow threads. That's quite a bit different from making the check-interval infinitely high for the duration of the block.