The web framework developed by Aaron Swartz and used when Reddit was rewritten in Python was web.py.
Reddit originally ran on Common Lisp, the language used by Steve Huffman for the site's first version. After Aaron Swartz's company Infogami merged with Reddit in late 2005, Swartz helped rewrite Reddit's codebase in Python. The rewrite used web.py, a lightweight web application framework Swartz had developed himself.
The change reflected practical engineering concerns. Python was considered easier for the growing team to read and maintain, while web.py supplied a small, direct toolkit for handling web requests and application logic. Swartz's work became part of Reddit's early technical history even though the exact corporate status of his role is sometimes described differently.
A common mistake is choosing Django, Flask, or Pylons. Those are real Python web frameworks, but they were not the framework identified with Swartz's early Reddit rewrite. web.py was created before Flask and is notably smaller and more minimalist than Django.