Binary search with print is done manually. If error not obvious from quick read, in a 20 line function, add print around line 10. If ok there, look down and add print later in function. If not, look up and add print earlier in function. This is not exact at all, but quick reruns (F5 with IDLE) mean no need to load up entire function with prints (that will have to be removed again) all at once (as might do with compiled C 20 years ago). Same idea as binary search through revisions to find buildbot breaker. I am sure you know this, even if not by terms I used.