I don't think the get_success_url method needs to change - it seems a pretty odd change to me anyway. The change to delete looks good though and makes sense.

Please take a look at https://code.djangoproject.com/ticket/19044
Change to get_success_url is necessary for example to redirect to object parent after its deletion

IMHO you shouldn't refer to an URL this way, you should use the reverse_lazy function here.

I have to agree with @mjtamlyn about the changes of the get_success_url method. You just can't assume that the specified success_url should be formatted using the instance's __dict__.

Why get_success_url of DeletionMixin cannot format url in the same way as get_success_url of ModelFormMixin in the same file?

Sorry, didn't realise it was a consistency thing. In that case the change should be there for consistency purposes, although I still think the feature smells ;)

@0x0badc0de please forgive me, didn't notice that. I wonder if it's documented somewhere.

Needs tests as noted on the ticket.

#663 has tests.