One more problem that adjusting of number of threads is performed exactly after placing the new task in the queue. In in some cases we can find that qsuze <> 0 but it doesn't mean that there is no idle threads. It can be equal to 1 (just queued task) as no threads manage to get it yet (don't forgot about GIL).

So even qsuze <> 0 dosn't mean that we need to create a new thread.

But (unfinished() <= num_threads) works perfect in this case also.