You actually don't need to know a whole lot of python to get started using
pygame and making simple games. The first game I made I didn't have any idea
about classes and functions and it got me pretty far. I would say you need
to know about lists, while loops, for loops and how to use them to loop over
lists.

My advice would be to forget about games like Double Dragon for the moment
and concentrate on a simple one like Pong. I know this might sound odd but
making a complete game, even if its something dead simple will boost your
confidence more then starting something hard and then having it take forever
to make. Just my experience anyway.