View mixins

Mixins are the essence of DRY code in class-based views. Like model mixins, a view mixin takes advantage of Python's multiple inheritance to easily reuse chunks of functionality. They are often parent-less classes in Python 3 (or derived from object in Python 2 since they are new-style classes).

Mixins intercept the processing of views at well-defined places. For example, most generic views use get_context_data to set the context dictionary. It is a good place to insert an additional context, such as a feed variable that points to all posts a user can view, as shown in the following command:

class FeedMixin(object):
    def get_context_data(self, **kwargs):
        context = super().get_context_data(**kwargs)
        context["feed"] = models.Post.objects.viewable_posts(self.request.user)
        return context

The get_context_data method first populates the context by calling its namesake in all the bases classes. Next, it updates the context dictionary with the...