It's definitely python, but definitely not Django. I recall someone explaining to me that it actually predates WSGI standardisation so its not even WSGI its just all custom python from start to finish.

My apologies, I thought OP said 'Python' not 'DJango'. You're correct. Youtube has their own framework and IIRC reddit runs a few different Pyramid modules rather than a full-fat framework.

Reddit has some scattered Engineering blogs and my insight into youtube comes from a previous HN article by the author of GrumPy (who works on youtube at Google) in case you're curious.