Episode 3 - Views On Django

On this episode, we look at views, a major component within Django and a primary place where your code will run.

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Last Episode

On the previous episode, we talked about URLs and how they describe the main interface that a browser can use to interact with your application.

What Is A View?

A view is a chunk of code that receives an HTTP request and returns an HTTP response. Views describe Django’s entire purpose: to respond to requests made to an application on the internet.

Function Views

A function view is exactly that, a function. The function takes an instance of HttpRequest as input and returns an HttpResponse (or one of its many subclasses) as output.

# application/views.py
from django.http import HttpResponse

def hello_world(request):
    return HttpResponse('Hello World')

HttpRequest

HttpRequest is a Python class. Instances of this class represent an HTTP request.

POST /courses/0371addf-88f7-49e4-ac4d-3d50bb39c33a/edit/ HTTP/1.1
Host: 0.0.0.0:5000
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: application/x-www-form-urlencoded
Content-Length: 155
Origin: http://0.0.0.0:5000
Connection: keep-alive
Upgrade-Insecure-Requests: 1
Pragma: no-cache
Cache-Control: no-cache

name=Science
&monday=on
&tuesday=on
&wednesday=on
&thursday=on
&friday=on

When Django receives a request like this, it will parse the data and store the data in an HttpRequest instance. Here are some common attributes.

  • method - This matches the HTTP method of POST and can be used to act on the kind of request the user sent.
  • content_type - This attribute instructs Django on how to handle the data in the request. The example value would be application/x-www-form-urlencoded to indicate that this is user submitted form data.
  • POST - For POST requests, Django processes the form data and stores the data into a dictionary-like structure. request.POST['name'] would be Science in our example.
  • GET - For a GET request, anything added to the query string (i.e., the content after a ? character such as student=Matt in /courses/?student=Matt is stored in a dictionary-like attribute as well.
  • headers - This is where all the HTTP headers like Host, Accept-Language, and the others are stored.

HttpResponse

The other major interface that your views will use either directly or indirectly is the HttpResponse interface.

Some of the common HttpResponse attributes include:

  • status_code - This is the HTTP status code. Status codes are a set of numbers that HTTP defines to tell a client (i.e., a browser) about the success or failure of a request. 200 is the usual success code. Any number from 400 and up will indicate some error like 404 when something is not found.
  • content - This is the content that you provide to the user. The response stores this data as bytes. If you supply Python string data, Django will encode to bytes for you.
>>> from django.http import HttpResponse
>>> response = HttpResponse('Hello World')
>>> response.content
b'Hello World'

When working with Django views, you won’t always use HttpResponse directly. Some common examples:

  • HttpResponseRedirect - You may want to send a user to a different page. Perhaps the user bought something on your site, and you would like them to see a receipt page of their order. This subclass is perfect for that scenario.
  • HttpResponseNotFound - This is the subclass used to create a 404 Not Found response. Django provides some helper functions to return this so you may not use this subclass directly, but it’s good to know it’s available.
  • HttpResponseForbidden - This type of response happens when you don’t want a user to access a part of your website.
  • JsonResponse - I haven’t focused on JSON yet in this series, but it’s a data format which matches closely to Python native data types and can be used to communicate with JavaScript.

Django has other techniques to return HttpResponse instances without creating one yourself. The most common function is render.

render is a tool for working with templates. Templates are the topic of the next article, but here is a sneak peek.

An undesirable way to send HTML:

from django.http import HttpResponse

def my_html_view(request):
    response_content = """
    <html>
    <head><title>Hello World!</title>
    <body>
        <h1>This is a demo.</h1>
    </body>
    </html>
    """
    return HttpResponse(response_content)

While this works, it has many shortcomings.

  1. The HTML chunk isn’t reusable by other views.
  2. The mixing of Python and HTML is going to get messy.
  3. How can you join pieces of HTML together?

The alternative:

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

def my_html_view(request):
    return render(request, "template.html", {})

And we would have another file named template.html containing:

<html>
<head><title>Hello World!</title>
<body>
    <h1>This is a demo.</h1>
</body>
</html>

That wraps up HttpRequest and HttpResponse.

View Classes

Views do no need to be functions exclusively. Django also provides tools to make views out of classes. These types of views derive from Django’s View class.

When you write a class-based view (often abbreviated to CBVs), you add class methods that match up with HTTP methods. Let’s see an example:

# application/views.py
from django.http import HttpResponse
from django.views.generic.base import View

class SampleView(View):
    def get(self, request, *args, **kwargs):
        return HttpResponse("Hello from a CBV!")

The get method on the class corresponds to a GET HTTP request method. This relationship holds for all the HTTP methods.

# project/urls.py
from django.urls import path

from application.views import SampleView

urlpatterns = [
    path("", SampleView.as_view()),
]

Note that we don’t pass SampleView to path as is. path expects a callable object so we must call as_view which is a class method that returns a function that will call the code in our class.

Django includes a host of class-based views to use for a variety of purposes. We can explore a few of them with our limited exposure to the full framework so far.

Out Of The Box Views

RedirectView

This is a view to use when you want to send users of your site to a different place. You could make a view that returns an HttpResponseRedirect instance, but this view can handle that for you.

In fact, you can use RedirectView without subclassing it. Check this out:

# project/urls.py
from django.urls import path
from django.views.generic.base import RedirectView

from application.views import NewView

urlpatterns = [

    path("new-path/", NewView.as_view(), name='new-view'),
    path("old-path/", RedirectView.as_view(pattern_name='new-view')),
]

as_view is what let’s us avoid subclassing RedirectView. The arguments passed to as_view override any class attributes.

TemplateView

Templates are so commonly used that Django provides a class that knows how to produce a proper response with nothing more than a template name.

An example looks like:

# application/views.py
from django.views.generic.base import TemplateView

class HomeView(TemplateView):
    template_name = 'home.html'

Other View Classes

Django’s other class-based views serve a variety of purposes. Django has views that will:

  • Display and handle HTML forms so users can input data and send the data to the application.
  • Pull data from a database and show an individual record to the user (e.g., a webpage to see facts about an individual movie).
  • Pull data from a database and show information from a collection of records to the user (e.g., showing the cast of actors from a movie).
  • Allow a user to create or update data that will be saved to a database.
  • Show data from specific time ranges like days, weeks, and months.

For now, when you’re developing your own views, try to remember that Django probably has a class-based view to aid your work.

Useful View Decorators And Mixins

Decorators are a feature of Python (and many other languages) that let you extend a function with additional capabilities. A decorator can wrap a view function to provide new behavior to a view. This is useful when you have common functionality that you want to add to many views without copying and pasting a lot of code around.

Mixin classes serves a very similar purpose as decorators, but behave with Python’s multiple inheritance feature of classes to “mix in” the new behavior with an existing class-based view.

Decorators To Know

What if you only want to handle certain HTTP methods? Consider:

# application/views.py
from django.http import HttpResponse

def multi_method_view(request):
    if request.method == 'GET':
        return HttpResponse('Method was a GET.')
    elif request.method == 'POST':
        return HttpResponse('Method was a POST.')

# application/views.py
from django.http import Http404, HttpResponse

def guard_clause_view(request):
    if request.method != 'POST':
        raise Http404()

    return HttpResponse('Method was a POST.')

# OR

def if_clause_view(request):
    if request.method == 'POST':
        return HttpResponse('Method was a POST.')
    else:
        raise Http404()

Instead, we can use the require_POST decorator and let Django check the method for us.

# application/views.py
from django.http import HttpResponse
from django.views.decorators.http import require_POST

@require_POST
def the_view(request):
    return HttpResponse('Method was a POST.')

Another common decorator you may encounter is the login_required decorator. When we get to the subject of user management, you’ll see that we can make a protected view for an app by including this decorator.

# application/views.py
from django.contrib.auth.decorators import login_required
from django.http import HttpResponse

@login_required
def the_view(request):
    return HttpResponse('This view is only viewable to authenticated users.')

A final example of a useful built-in decorator is user_passes_test. This is another decorator used with the user management system that let’s us control which users should be allowed to access a view. For instance, we could make a view that only staff-level users could access.

# application/views.py
from django.contrib.auth.decorators import user_passes_test
from django.http import HttpResponse

@user_passes_test(lambda user: user.is_staff)
def the_view(request):
    return HttpResponse('Only visible to staff users.')

And, they stack!

# application/views.py
from django.contrib.auth.decorators import user_passes_test
from django.http import HttpResponse
from django.view.decorators.http import require_POST

@require_POST
@user_passes_test(lambda user: user.is_staff)
def the_view(request):
    return HttpResponse('Only staff users may POST to this view.')

Mixins To Know

Mixin classes are to class-based views as decorators are to function-based views. This isn’t completely true since class-based views can also use decorators, but it should give you an idea of where mixins fit.

Like the login_required and user_passes_test decorators, we have mixin equivalents of LoginRequiredMixin and UserPassesTestMixin. Maybe you have some template views that should only be accessible to authenticated users or staff-level users. Those views could look like:

# application/views.py
from django.contrib.auth.mixins import LoginRequiredMixin, UserPassesTestMixin
from django.views.generic.base import TemplateView

class HomeView(LoginRequiredMixin, TemplateView):
    template_name = 'home.html'

class StaffProtectedView(UserPassesTestMixin, TemplateView):
    template_name = 'staff_eyes_only.html'

    def test_func(self):
        return self.request.user.is_staff

There are plenty of other mixin classes. In fact, most of Django’s built-in class-based views are constructed by composing various mixin classes together. If you’d like to see how they are constructed, check out Classy Class-Based Views which is a site showing the built-in CBVs along with the mixins and attributes available to those classes.

Summary

That’s a wrap on view fundamentals. We’ve looked at:

  • View functions
  • HttpRequest and HttpResponse
  • View classes
  • Some built-in supporting views
  • Decorators and mixins that supercharge views.

Next Time

In the next episode, we’ll examine templates in more depth. Templates are a sub-language within Django that are your primary tool for making user interfaces with Django’s built-in features.

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Newer

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Episode 4 - Building User Interfaces

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On this episode, we discuss Django’s front door, URLs. We talk about what URLs are, how to build them in Django, and the functions Django provides to work with URLs.