Update: I am currently unable to get any of this to work on the yasnippet 0.6.x release series. Setting yas/trigger-key seems to not do anything or throws an integer-or-marker-p error, and then even when I leave that out, yas/expand hangs for half a second before declaring that it cannot find any snippets. This grumble was written with yasnippet 0.5.5 installed. If anyone knows what’s happening, drop a comment.
You can stick this in your yasnippet (Emacs templating) pipe and chew on it for a while: "cls" "class ${class}(${object}):$0".