I have found that the most reliable way I like is to just construct the command externally and then pass to gnu parallel (mostly for --eta and --tmuxpane). And the great thing is, as others say, xargs -I. I prefer for shortness (and few collisoins), '@'

seq 1 10|xargs -I@ echo 'bash run.py @'|parallel -j 10

I know the echo is a little silly but then I can remove the |parallel and see if it's right. And if I don't want parallelism, I just pass to bash