A message from Guido van Rossum on the Python-Dev mailing list confirms this (http://mail.python.org/pipermail/python-dev/2006-December/07...). YouTube was launched in 2005 and the message is from 2006, so it’s a safe bet they’ve been Python-based right along.
> “Python is fast enough for our site and allows us to produce maintainable features in record times, with a minimum of developers”—Cuong Do, Software Architect, YouTube.com
http://paulbuchheit.blogspot.com/2007/04/secret-to-making-th...