Django edit form based on add form?

I've made a nice form, and a big complicated 'add' function for handling it. It starts like this...

def add(req):
    if req.method == 'POST':
        form = ArticleForm(req.POST)
        if form.is_valid():
            article = form.save(commit=False)
            article.author = req.user
            # more processing ...

Now I don't really want to duplicate all that functionality in the edit() method, so I figured edit could use the exact same template, and maybe just add an id field to the form so the add function knew what it was editing. But there's a couple problems with this

1. Where would I set article.id in the add func? It would have to be after form.save because that's where the article gets created, but it would never even reach that, because the form is invalid due to unique constraints (unless the user edited everything). I can just remove the is_valid check, but then form.save fails instead.
2. If the form actually is invalid, the field I dynamically added in the edit function isn't preserved.

So how do I deal with this?