Django Best Practices: Function-Based Views vs Class-Based Views

Django's use of both function-based views (FBVs) and class-based views (CBVs) causes a lot of confusion for newcomers. Why have multiple ways to do the same thing? And which one should I use? This article will cover the history of FBVs and CBVs in Django and cover the pros and cons of each approach.

In a Django project the order of operations is generally urls.py -> views.py -> models.py + template.html. An HTTP request is made to a URL like example.com. Within Django a project-level urls.py file redirects the request to an appropriate views.py file which includes a template.html file and models.py file (if needed) along with additional logic in the view itself. The view then returns an HTTP response to the user.

This process happens over and over again for each webpage within a Django site.

Background

Historically Django used only function-based views (FBV). These take a standard Python function form of accepting a request object as an argument and returning a response object. Everything is visible in one place. Here is an example of a function that returns a simple template. For simplicity, no database models are included here.

# helloworld/views.py
from django.shortcuts import render

def homepageView(request):
    return render(request, "helloworld/index.html")

Notice that the view accepts a single request object as the argument and uses the built-in render shortcut to return a response. In this case, it displays the template index.html within the helloworld app.

FBVs can often grow quite large in their size because all the logic for the view must be included. The upside of this approach is that there is no hidden behavior; you know exactly what you're getting. Decorators can be added, yes, but largely what you see is what you get. The downside is that FBVs can often become quite long--15 or even 30+ lines isn't unheard of--which is difficult to reason about. And since most web functionality is essentially the same there is a lot of repeated code that could be abstracted away.

The downside is length and repetition. How readable is a 15-line FBV? How about a 30-line FBV? Especially when probably 12 of the lines are doing the exact same work as other FBVs and really there's only 3 lines that are different. As a result, 14 function-based generic views were added to handle common use cases.

Class-based Views (CBVs)

Starting with Django 1.3, class-based views were added that used inheritance to write much more concise views. Generic class-based views were also added and deliberately mimicked earlier function-based generic views.

Using a Generic CBV requires understanding the inheritance structure which is a higher learning curve since it is not as readily apparant as it is in a FBV. However it is possible to understand the structure with practice and tools like Classy Class-Based Views. The upsides are no more code repetition and much more concise views that make it easier to focus on what's been changed not repeat generic code.

The winner?

So which one to use? Personally, I always start with a GCBV if possible, drop down to a CBV if needed, and if I really must get granular with behavior I'll use a FBV on occasion. Most--but not all--Django developers I know follow a similar approach. Especially since generic function-based views no longer exist!

It's worth noting that the Django source code itself uses CBVs and via mixins more and more behavior that in the past required a FBV is becoming available within a CBV context.

I do think, however, that once you've hit an intermediate level of comfort with Django, the ability to write the same view both as a FBV and as a CBV is valuable. Arguably it is easier from a learning perspective to see all the logic in one place via a FBV versus having to go through the inheritance chain of a CBV.

So FBVs have their place and it's a strength of Django, in my opinion, that both FBVs and CBVs are supported. Unfortunately this comes at the cost of some additional confusion for newcomers. Hence why I wrote this post :).

Not all Django developers agree though. There are very experienced and vocal proponents of function-based views who consider class-based views to be a mistake. Luke Plant wrote a very extensive series of articles called Django Views - The Right Way arguing this very point. It is highly recommended to read it. To truly master Django it is necessary to be comfortable with both FBVs and CBVs and be aware of their relative pros and cons.

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