Does know any python video editing/processing libraries or frameworks that work with Google App Engine? I need to pro grammatically edit video files (adding a soundtrack, layering video clips on top etc.) on my App Engine site.
With most video/audio codecs written in C for maximum portability, and any muxing libraries being written in C for speed working with large byte streams, finding these features in pure python libraries is going to be difficult.
If you could tie yourself down to just a single codec/container format you could probably port something to pure python but you're in for a long slog if you want to build something as diverse as ffmpeg in pure python.
I would not recommend this sort of task for AppEngine, even with backends, I think you would be severally limiting yourself by forcing this on AppEninge as well as making your task much harder than it would be if you looked at alternative solutions.
I would get myself a VPS or EC2 instance, build a little webservice around ffmpeg and send tasks to it from my main AppEngine application.