(I decided to risk publishing this as an answer, not a comment).
The three “reliable” candidates for editing / processing video in Python that I found are PyMedia (which I gave up), PyGame (which can satisfy your needs, but doesn't match mine) and pyffmpeg (which is very promising but depends on Cython, and I could not get Cython to work on my system).
Most mature video editing or processing programs (commercial or open source) use the FFmpeg libraries in some way. For example, PyMedia includes a modified version of libav * libraries. Even many commercial tools use these libraries in violation of the FFmpeg license.
FFmpeg is implemented in C and does not have hooks for other languages out of the box. Learning the APIs for libav * libraries is not for the faint of heart (without direct access to documents, just read the source, examples, and frequently asked questions), but, as far as I can tell, FFmpeg basically lies in how the video is made into the "real world".
So, if you cannot find a ready-made solution for Python that suits your needs, you can use SWIG or another interface generator to wrap the FFmpeg stock libraries as a starting point. I mean to do it myself in the future, if I find the time.
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