Change in behavior for optional re matches

[Terence Honles]

I am mainly pointing this out as it probably should have been called out as a breaking change from Django 2.X to 3.X (And I think the release notes should probably be adjusted if the old functionality is not to be restored). You can probably argue either way about if this is a bug since it's coming from re's match.groupdict(). It's related to https://code.djangoproject.com/ticket/31061 and caused by https://code.djangoproject.com/ticket/26431 / ​https://github.com/django/django/pull/11477

The test is the following:

from django.urls import re_path

def view(request, optional1, optional2): ...

urlpatterns = [
  re_path(r'^(?P<optional1>\w+/)(?:(?P<optional2>\w+)/)', view)
]

For Django 2.X this would work as expected because named captures are returned even if they are None, however in Django 3.0 this would raise an error because positional parameter optional2 is no longer provided. (i.e. view() missing 1 required positional argument: 'optional2')

You can argue that optional2 should actually have had a default all along, but it didn't before and code may unexpectedly break because it is now required.

I'm not very familiar with https://code.djangoproject.com/ticket/26431, but I think if possible it probably should have been fixed at a different level.