It would be better if you collected all the arguments passed and send them directly to `git checkout`, so the user can use this command as if it were a regular `git checkout` call, instead you always add the pull request reference to the list of arguments (first). That you allow users even to check out a single file from a pull request 8-)

See http://docs.python.org/2.7/library/argparse.html#nargs, the `argparse.REMAINDER` is what you're looking for...