For user-friendlyness we should also have a branch that includes the case where django_filters isn't is installed so thatFilterSet imports still succeed, but raise an assertion error letting the user know that they need to installdjango-filter.

This could just be...

if django_filters:    ...else:    def FilterSet(*args, **kwargs):        assert False, 'django-filter must be installed to use the `FilterSet` class'

Makes sense. I've added a commit for this. It's a bit complicated since FilterSet is designed to be subclassed, so getting a nice error message at the point of use needed another metaclass.

Wondering if it'd be marginally cleaner to haveFilterSet declarationinside theif case, rather thanafter it.

Then theelse case could just have aFilterSet stub that raises an assertion error, rather than having aFilterSet metaclass.

The metaclass is required if we want the assert to fire whenFilterSet is subclassed. That way the context of the error is at the actual point of use. Another alternative is a dummy class with an assert in__init__, but then the error won't happen until you hit the view.