I use the cgi module to parse the request, specifically the query string. This is one of cgi's undocumented features (well, it's only documented in cgi.py, in the docstrings). I pass it an explicit environment (which includes the query string) and file-in. It will work quite well with mod_python -- it's not using stdin or os.environ. Though an application coded for mod_python might be a bit harder, as such an application might use mod_python features that WSGI doesn't include.
It's expected that you won't code your application to WSGI directly, but use some framework or library to make it a bit easier to work with. The cgi is one such library, though it's not a very good one.