In the previous Understand Django article, we got deep into the Django administrators site. We saw what the site was and how to configure and customize it. In this article, we will examine what goes into an application. Applications are core elements of a Django project.
- From Browser To Django
- URLs Lead The Way
- Views On Views
- Templates For User Interfaces
- User Interaction With Forms
- Store Data With Models
- Administer All The Things
- Anatomy Of An Application
- User Authentication
- Middleware Do You Go?
- Serving Static Files
- Test Your Apps
- Deploy A Site Live
- Per-visitor Data With Sessions
- Making Sense Of Settings
- User File Use
- Command Your App
- Go Fast With Django
- Security And Django
- Debugging Tips And Techniques
What Is An Application?
Before getting to what a Django application is, we probably need to start with what it is not because the terminology is confusing. In the world of web development, developers may call a website a “web application.”
In Django parlance, a “web application” is a Django project. All of the pieces that come together to make a website are a project. The primary components within the project are called applications. In other words, a Django project is built from one or more Django applications.
This situation is quite similar
to Python packages.
The software industry
often describes the software unit
as a “package.”
as “package” managers.
This leads to a similar naming problem
because Python also calls any directory
__init__.py file a “package” as well.
In reality, the code that you download using pip is technically called a “distribution.” Even though we colloquially talk about Python downloads from PyPI (Python Package Index) as packages, we’re really talking about distributions, and a distribution is a unit that contains one or more Python packages.
Hopefully, you now understand the relationship of applications in Django.
Your “web application” is a Django project composed of one or more Django applications.
Let’s look at a fully loaded Django application to see the fairly standard structure that you will encounter in Django projects.
An application usually tries to capture a core concept within your system. For this article, we will use movies as the concept we want to model.
Let’s see what a default scaffold includes, then build it up with all the extras.
(venv) $ ./manage.py startapp movies (venv) $ tree movies movies ├── __init__.py ├── admin.py ├── apps.py ├── migrations │ └── __init__.py ├── models.py ├── tests.py └── views.py
This file is where all your
ModelAdmin classes live
to power how the movies app will appear
in the Django admin.
You can learn more about the admin
in Administer All The Things.
This file is for the
of the application.
We will discuss the
and how to use it
later in this article.
This directory is where all database migrations are stored
for the application.
Any model changes for this app will generate a migration
and create a numbered migration file
in this directory.
More info about migrations is
in Store Data With Models.
This file is the home
for all the Django
in the application.
The models represent all your database data.
Learn more about models
in Store Data With Models.
This file is for automated tests.
We’ll cover automated tests
in a future article.
you can know that I always delete this file
and replace it with a
tests package is superior
because you can split out to more focused files
to know where the appropriate tests are.
This file is where Django view functions or classes go.
Views are the glue code
that connect your URL routes
to your database models.
I wrote about views
in Views On Views.
That’s everything that comes with a generated app, but what other files are missing that you will commonly see in a Django application?
This file is often used
to create routes
that logically group
all movie related functionality.
urls.py file would power all the routes
on URLs is
in URLs Lead The Way.
When you use Django
to interact with users,
this is the file
where forms are stored.
You can discover more on forms
in User Interaction With Forms.
This directory is a Python package
that would include a module
where you’d define any custom template tags
to use when rendering your templates.
Custom tags are a topic
in Templates For User Interfaces.
This directory can store templates
that the application will render.
I personally prefer
using a project-wide
in Templates For User Interfaces,
templates directories are commonly found
in individual Django apps,
especially for third party applications
that you may pull into your project.
For static files
that you want to display,
such as images,
you can use the
We’ll discuss static files more
in a future article.
Users can extend Django
with custom commands
that can be called via
Those commands are stored
in this package.
Custom commands are a future topic
in this series.
When doing translations and internationalization,
the translation files must have a home.
That’s the purpose
This file is not always used,
but if your application has a lot
of custom managers,
then you may want
to separate them
from your models
in this file.
Managers are a topic
in Store Data With Models.
Most applications will not have all of these pieces, but this should give you an idea of what things are when you are exploring Django apps in the wild on your own. Here’s what our final sample tree would look like.
(venv) $ tree movies movies ├── __init__.py ├── admin.py ├── apps.py ├── forms.py ├── locale │ └── es │ └── LC_MESSAGES │ ├── django.mo │ └── django.po ├── management │ ├── __init__.py │ └── commands │ ├── __init__.py │ └── do_movie_stuff.py ├── managers.py ├── migrations │ ├── 0001_initial.py │ └── __init__.py ├── models.py ├── static │ └── movies │ └── moviereel.png ├── templates │ └── movies │ ├── index.html │ └── movie_detail.html ├── templatestags │ ├── __init__.py │ └── movies_tags.py ├── tests │ ├── __init__.py │ ├── test_models.py │ └── test_views.py ├── urls.py └── views.py
We’ve now seen what’s in a Django application and have an idea of an app’s composition. How does Django load applications?
Django does not do automatic discovery
of Django applications
within your project.
If you want Django
to include an app
in your project,
you must add the app
in the settings file.
This is a good example of Django following the Python ethos of favoring explicit over implicit. By being explicit, your project is not in danger of including apps that you don’t expect. That might seem silly for apps that you write yourself, but you’ll be thankful that’s the case when some third party package in your virtual environment happens to have a Django app that you don’t want in your project.
when an application is
Django will look
This class is stored
and contains metadata
about the application.
When Django starts, it will initialize the system by doing the following:
- Load the settings
- Configure logging (a topic we’ll explore in the future)
- Initialize an application registry
- Import each package from the
- Import a models module for each application
- Invoke the
readymethod of every
ready method is a useful hook
for taking action
Since models are already loaded
by the time the method is called,
it’s a safe place
to interact with Django.
If you attempt to run setup code
before Django is ready,
and you try to do something
like use the ORM
to interact with database data,
you’ll probably get an
Most apps won’t directly need to run startup code,
but knowing about the
is a useful piece
to keep in your back pocket.
An application is an important tool for grouping the different logical components in your project, but apps also serve another purpose. Apps are the basis for most of the 3rd party extensions in the Django ecosystem.
A big reason to use Django is that the framework takes a “batteries included” approach. Most of the tools that you need to build a website are baked directly into the framework. This is a vastly different approach compared to Flask which provides a relatively small API and depends heavily on third party libraries.
Even though Django includes most of the major pieces for a web application, the framework doesn’t include everything. When you want to include more features, Django apps fill in the gaps.
Before you go out to PyPI,
we need look no further
a collection of “contributed” applications provided
by Django itself.
When you run the
Django will include a variety
of built-in applications
that perform different functions.
If you don’t need some of the functionality,
you can opt out
by removing the app
from your list
I think this is the big difference in philosophy behind the framework. Some developers like to start from an extremely minimal kernel of functionality and build it up based on their needs. Django’s philosophy seems to be that you start with an opinionated baseline and pare down what you don’t require. Django doesn’t expect that you’ll use every feature in every app, but many of the features that you’ll want are at the ready when you need them.
From my point of view, I think the Django philosophy is the right one (shocking, isn’t it!? 🤪). The benefit of the Django philosophy is that you leverage the knowledge of people who have built web apps for a very long time. Not only do you leverage that knowledge, you benefit from the polishing applied by the Django developers to integrate the different major systems into a consistent whole. What you’re left with is a framework that feels like it belongs together, and I think that is a positive impact on your productivity.
When you build from a minimal kernel and work up, you depend on knowing everything that’s required to put something on the web. That means that you know all the pieces and how to bolt them together. But most people don’t know all the pieces (because there are so many!).
If you start minimally and don’t know the pieces, you’ll learn along the way, but what happens when you encounter a new concept that doesn’t fit into your original mental model? For instance, security is a critical part that can destroy your mental model when you learn of a class of vulnerabilities that can restrict what is possible to do safely. When you follow this building from scratch approach, I think the final result will naturally be your own custom framework. If that’s your thing, awesome. Go for it. For me, I want a framework that is a commodity and commonly understood by many people.
Ok, so, what does this have to do with Django apps? Apps are contained and reusable modules. Because they have a fairly standard structure, a project can integrate a new app quickly. This means you can leverage the knowledge and experience (read: battle scars) of other web developers. The apps all play by the same rules so you, as the developer, spend less time gluing the app into your project and more time benefiting from what it does.
I think this standard structure also makes it easier
with new apps.
When I need some new functionality,
I will often check
that could meet my needs.
In my experience,
adding a new app is,
in many cases,
than installing the package,
adding the app
and putting an
Some packages require more configuration
but I think
that the integration cost is low enough
for me to experiment rapidly
and back out my decision
if I discover
that an app won’t do
what I need.
All in all, Django applications make working with the Django ecosystem a more enjoyable experience.
In this article, we studied Django applications.
- What a Django application is
- How a Django application is structured
- How the Django ecosystem benefits from a common format that creates reusable components
Next time we will look into authentication in Django. We will study:
- How users are created and managed
- How to deal with permissions for users
- How to work with users in your views and templates
If you’d like to follow along with the series, please feel free to sign up for my newsletter where I announce all of my new content. If you have other questions, you can reach me online on Twitter where I am @mblayman.