Django Design Patterns and Best Practices - Second Edition

5 (8 reviews total)
By Arun Ravindran
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  1. Django and Patterns

About this book

Building secure and maintainable web applications requires comprehensive knowledge. The second edition of this book not only sheds light on Django, but also encapsulates years of experience in the form of design patterns and best practices. Rather than sticking to GoF design patterns, the book looks at higher-level patterns. Using the latest version of Django and Python, you’ll learn about Channels and asyncio while building a solid conceptual background. The book compares design choices to help you make everyday decisions faster in a rapidly changing environment.

You’ll first learn about various architectural patterns, many of which are used to build Django. You’ll start with building a fun superhero project by gathering the requirements, creating mockups, and setting up the project. Through project-guided examples, you’ll explore the Model, View, templates, workflows, and code reusability techniques. In addition to this, you’ll learn practical Python coding techniques in Django that’ll enable you to tackle problems related to complex topics such as legacy coding, data modeling, and code reusability.

You’ll discover API design principles and best practices, and understand the need for asynchronous workflows. During this journey, you’ll study popular Python code testing techniques in Django, various web security threats and their countermeasures, and the monitoring and performance of your application.

Publication date:
May 2018
Publisher
Packt
Pages
282
ISBN
9781788831345

 

Chapter 1. Django and Patterns

In this chapter, we will talk about the following topics:

  • Why Django?
  • The story of Django
  • How does Django work?
  • What is a pattern?
  • Well-known pattern collections
  • Patterns in Django

According to Bowei Gai's World Startup Report, there were more than 136,000 internet firms across the world in 2013, with more than 60,000 in America alone. Of these, 87 US companies are valued at more than $1 billion. Another study says that of 12,000 people aged between 18 years and 30 years in 27 countries, more than two-thirds see opportunities to become an entrepreneur.

This entrepreneurial boom in digital startups is primarily due to the tools and technologies of startups becoming cheap and ubiquitous. Creating a full-fledged web application takes a lot less time and skill than it used to, thanks to powerful frameworks.

Physicists, educators, artists, and many others without a software engineering background are creating useful applications that are significantly advancing their domains. However, they may not be aware of the software engineering design principles needed to construct large and maintainable software.

A study of four different implementations of a web-based application in Norway showed implementations with known code smells and design anti-patterns to be directly associated with the difficulties in maintenance. Poorly designed software might work just as well but can be difficult to adapt to evolving requirements in a fast-changing world.

Beginners often discover design issues late in their project. Soon, they would attempt to solve the same problems others have faced again and again. This is where understanding patterns can really help save their time.

 

Why Django?


Every web application is different, like a piece of handcrafted furniture. You will rarely find a mass-produced sofa meeting all your needs perfectly. Even if you start with a basic requirement, such as a blog or social network, your needs will slowly grow, and you can easily end up with a lot of half-baked solutions duct-taped onto a once simple cookie cutter solution.

This is why web frameworks, such as Django or Rails, have become extremely popular. Frameworks speed up development and have all the best practices baked in. However, they are also flexible enough to give you access to just enough plumbing for the job. Today, web frameworks are ubiquitous, and most programming languages have at least one end-to-end framework similar to Django.

Python probably has more web frameworks than most programming languages. A quick look at Python Package Index (PyPI) brings up an amazing 13,045 packages related to web environments. For Django, the total is 9,091 packages. The Python wiki lists over 54 active web frameworks with the most popular ones being Django, Flask, Pyramid, and Zope. Python also has a wide diversity in frameworks. The compact bottle micro web-framework is just one Python file that has no dependencies and is surprisingly capable of creating a simple web application.

Despite these abundant options, Django has emerged as a big favorite by a wide margin. Djangosites.org lists over 5,263 sites written in Django, including famous success stories such as Instagram, Pinterest, and Disqus. As the official description says, Django (https://djangoproject.com) is a high-level Python web framework that encourages rapid development and clean, pragmatic design. In other words, it is a complete web framework with batteries included just like Python.

The out-of-the-box admin interface, one of Django's unique features, is extremely helpful for early data entry and administration. Django's documentation has been praised for being extremely well-written for an open source project.

Finally, Django has been battle-tested in several high traffic websites. It has an exceptionally sharp focus on security with protection against common attacks such as Cross-site scripting (XSS), Cross-site request forgery (CSRF) to evolving security threats such as weak password hashing algorithms.

Although you can use Django to build any kind of web application in theory, it might not be the best for every use case. For example, to prototype a simple web service in an embedded system with tight memory constraints, you might want to use Flask, while you might eventually move to Django for its robustness and features. Choose the right tool for the job.

Some of the built-in features, such as the admin interface, might sound odd if you are used to other web frameworks. To understand the design of Django, let's find out how it came into being.

 

The story of Django


When you look at the Pyramids of Egypt, you would think that such a simple and minimal design must have been quite obvious. In truth, they are the products of 4,000 years of architectural evolution. Step Pyramids, the initial (and clunky) design, had six rectangular blocks of decreasing size. It took several iterations of architectural and engineering improvements until the modern, glazing, and long-lasting limestone structures were invented.

Looking at Django, you might get a similar feeling — so elegantly built, it must have been flawlessly conceived. On the contrary, it was the result of rewrites and rapid iterations in one of the most high-pressure environments imaginable — a newsroom!

In the fall of 2003, two programmers, Adrian Holovaty and Simon Willison, working at the Lawrence Journal-World newspaper, were working on creating several local news websites in Kansas. These sites, including LJWorld.com, Lawrence.com, and KUsports.com, like most news sites were not just content-driven portals chock-full of text, photos, and videos, but they also constantly tried to serve the needs of the local Lawrence community with applications, such as a local business directory, events calendar, and classifieds.

A framework is born

This, of course, meant lots of work for Simon, Adrian, and later Jacob Kaplan Moss who had joined their team; with very short deadlines, sometimes with only a few hours' notice. Since it was the early days of web development in Python, they had to write web applications mostly from scratch. So, to save precious time, they gradually refactored out the common modules and tools into something called The CMS.

Eventually, the content management parts were spun off into a separate project called the Ellington CMS, which went on to become a successful commercial CMS product. The rest of The CMS was a neat underlying framework that was general enough to be used to build web applications of any kind.

By July 2005, this web development framework was released as Django (pronounced Jang-Oh) under an open source Berkeley Software Distribution (BSD) license. It was named after the legendary jazz guitarist Django Reinhardt. And the rest, as they say, is history.

Removing the magic

Due to its humble origins as an internal tool, Django had a lot of Lawrence Journal-World-specific oddities. To make Django truly general purpose, an effort dubbed Removing the Lawrence had already been underway.

However, the most significant refactoring effort that Django developers had to undertake was called Removing the Magic. This ambitious project involved cleaning up all the warts Django had accumulated over the years, including a lot of magic (an informal term for implicit features) and replacing them with a more natural and explicit Pythonic code. For example, the model classes used to be imported from a magic module called django.models.*, rather than being directly imported from the models.py module they were defined in.

At that time, Django had about a hundred thousand lines of code, and it was a significant rewrite of the API. On May 1, 2006, these changes, almost the size of a small book, were integrated into Django's development version trunk and released as Django release 0.95. This was a significant step toward the Django 1.0 milestone.

Django keeps getting better

Every year, conferences called DjangoCons are held across the world for Django developers to meet and interact with each other. They have an adorable tradition of giving a semi-humorous keynote on why Django sucks. This could be a member of the Django community, or someone who works on competing web frameworks or just any notable personality. Over the years, it is amazing how Django developers took these criticisms positively and mitigated them in subsequent releases.

Here is a short summary of the improvements corresponding to what once used to be a shortcoming in Django and the release they were resolved in:

  • New form-handling library (Django 0.96)
  • Decoupling admin from models (Django 1.0)
  • Multiple database supports (Django 1.2)
  • Managing static files better (Django 1.3)
  • Better time zone support (Django 1.4)
  • Customizable user model (Django 1.5)
  • Better transaction handling (Django 1.6)
  • Built-in database migrations (Django 1.7)
  • Multiple template engines (Django 1.8)
  • Simplified URL routing syntax (Django 2.0)

Over time, Django has become one of most idiomatic Python codebases in the public domain. Django source code is also a great place to learn the architecture of a large Python web framework.

 

How does Django work?


To truly appreciate Django, you will need to peek under the hood and see the various moving parts inside. This can be both enlightening and overwhelming. If you are already familiar with the following information, you might want to skip this section:

How web requests are processed in a typical Django application

The preceding diagram shows the simplified journey of a web request from a visitor's browser to your Django application and back. The numbered paths are as follows:

  1. The browser sends the request (essentially, a string of bytes) to your web server.
  2. Your web server (say, Nginx) hands over the request to a Web Server Gateway Interface (WSGI) server (say, uWSGI) or directly serves a file (say, a CSS file) from the filesystem.
  3. Unlike a web server, WSGI servers can run Python applications. The request populates a Python dictionary called environ and, optionally, passes through several layers of middleware, ultimately reaching your Django application.
  4. URLconf (URL configuration) module contained in the urls.py of your project selects a view to handle the request based on the requested URL. The request has turned into HttpRequest, a Python object.
  1. The selected view typically does one or more of the following things:

a. Talks to a database via the models

b. Renders HTML or any other formatted response using templates

c. Returns a plain text response (not shown)

d. Raises an exception

  1. The HttpResponse object gets rendered into a string, as it leaves the Django application.
  2. A beautifully rendered web page is seen in your user's browser.

Though certain details are omitted, this representation should help you appreciate Django's high-level architecture. It also shows the roles played by the key components, such as models, views, and templates. Many of Django's components are based on several well-known design patterns.

 

What is a pattern?


What is common between the words blueprint, scaffolding, and maintenance? These software development terms have been borrowed from the world of building construction and architecture. However, one of the most influential terms comes from a treatise on architecture and urban planning written in 1977 by the leading Austrian architect Christopher Alexander and his team consisting of Murray Silverstein, Sara Ishikawa, and several others.

The term pattern came in vogue after their seminal work, A Pattern Language: Towns, Buildings, Construction (volume 2 in a five-book series), based on the astonishing insight that users know about their buildings more than any architect ever could. A pattern refers to an everyday problem and its proposed but time-tested solution.

In the book, Christopher Alexander states the following:

"Each pattern describes a problem, which occurs over and over again in our environment, and then describes the core of the solution to that problem in such a way that you can use this solution a million times over, without ever doing it the same way twice."

For example, his wings of light pattern describe how people prefer buildings with more natural lighting and suggests arranging the building so that it is composed of wings. These wings should be long and narrow, never more than 25 feet wide. Next time you enjoy a stroll through the long well-lit corridors of an old university, be grateful to this pattern.

Their book contained 253 such practical patterns, from the design of a room to the design of an entire city. Most importantly, each of these patterns gave a name to an abstract problem and together formed a pattern language.

Remember when you first came across the word déjà vu? You probably thought: "wow, I never knew that there was a word for that experience." Similarly, architects were not only able to identify patterns in their environment but could also, finally, name them in a way that their peers could understand.

In the world of software, the term design pattern refers to a general repeatable solution to a commonly occurring problem in software design. It is a formalization of best practices that a developer can use. Like in the world of architecture, the pattern language has proven to be extremely helpful to communicate a certain way of solving a design problem to other programmers.

There are several collections of design patterns, but some have been considerably more influential than the others.

Gang of four patterns

One of the earliest efforts to study and document design patterns was a book titled Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, who later became known as the Gang of Four (GoF). This book is so influential that many consider the 23 design patterns in the book as fundamental to software engineering itself.

In reality, the patterns were written primarily for static object-oriented programming languages, and it had code examples in C++ and Smalltalk. As we will see shortly, some of these patterns might not even be required in other programming languages with better higher-order abstractions such as Python.

The 23 patterns have been broadly classified by their type as follows:

  • Creational patterns: These include abstract factory, builder pattern, factory method, prototype pattern, and singleton pattern
  • Structural patterns: These include adapter pattern, bridge pattern, composite pattern, decorator pattern, facade pattern, flyweight pattern, and proxy pattern
  • Behavioral patterns: These include chain-of-responsibility, command pattern, interpreter pattern, iterator pattern, mediator pattern, memento pattern, observer pattern, state pattern, strategy pattern, template pattern, and visitor pattern

While a detailed explanation of each pattern would be beyond the scope of this book, it would be interesting to identify some of these patterns present in Django implementation itself:

GoF Pattern

Django Component

Explanation

Command pattern

HttpRequest

This encapsulates a request in an object

Observer pattern

Signals

When one object changes state, all its listeners are notified and updated automatically

Template method

Class-based generic views

Steps of an algorithm can be redefined by subclassing without changing the algorithm's structure

While these patterns are mostly of interest to those studying the internals of Django, the most commonly question asked is, under which pattern is Django itself classified?

Is Django MVC?

Model-View-Controller (MVC) is an architectural pattern invented by Xerox PARC in the 70s. Being the framework used to build user interfaces in Smalltalk, it gets an early mention in the GoF book.

Today, MVC is a very popular pattern in web application frameworks. A variant of the common question is whether Django is an MVC framework.

The answer is both yes and no. The MVC pattern advocates the decoupling of the presentation layer from the application logic. For instance, while designing an online game website API, you might present a game's high scores table as an HTML, XML, or comma-separated values (CSV) file. However, its underlying model class would be designed independently of how the data would be finally presented.

MVC is very rigid about what models, views, and controllers do. However, Django takes a much more practical view to web applications. Due to the nature of the HTTP protocol, each request for a web page is independent of any other request. Django's framework is designed like a pipeline to process each request and prepare a response.

Django calls this the Model-Template-View (MTV) architecture. There is a separation of concerns between the database interfacing classes (model), request-processing classes (view), and a templating language for the final presentation (template).

If you compare this with the classic MVC — a model is comparable to Django's Models; a view is usually Django's Templates, and the controller is the framework itself that processes an incoming HTTP request and routes it to the correct view function.

If this has not confused you enough, Django prefers to name the callback function to handle each URL a view function. This is, unfortunately, not related to the MVC pattern's idea of a view.

Fowler's patterns

In 2002, Martin Fowler wrote Patterns of Enterprise Application Architecture, which described 40 or so patterns he often encountered while building enterprise applications.

Unlike the GoF book, which described design patterns, Fowler's book was about architectural patterns. Hence, they describe patterns at a much higher level of abstraction and are largely programming language agnostic.

Fowler's patterns are organized as follows:

  • Domain logic patterns: These include domain model, transaction script, service layer, and table module
  • Data source architectural patterns: These include row data gateway, table data gateway, data mapper, and active record
  • Object-relational behavioral patterns: These include Identity Map, Unit of Work, and Lazy Load
  • Object-relational structural patterns: These include Foreign Key Mapping, Mapping, Dependent Mapping, Association Table Mapping, Identity Field, Serialized LOB, Embedded Value, Inheritance Mappers, Single Table Inheritance, Concrete Table Inheritance, and Class Table Inheritance
  • Object-relational metadata mapping patterns: These include Query Object, Metadata Mapping, and repository
  • Web presentation patterns: These include Page Controller, Front Controller, Model View Controller, Transform View, Template View, Application Controller, and Two-Step View
  • Distribution patterns: These include Data Transfer Object and Remote Facade
  • Offline concurrency patterns: These include Coarse-Grained Lock, Implicit Lock, Optimistic Offline Lock, and Pessimistic Offline Lock
  • Session state patterns: These include Database Session State, Client Session State, and Server Session State
  • Base patterns: These include Mapper, Gateway, Layer Supertype, Registry, Value Object, Separated Interface, Money, Plugin, Special Case, Service Stub, and Record Set

Almost all of these patterns would be useful to know while architecting a Django application. In fact, Fowler's website at http://martinfowler.com/eaaCatalog/ has an excellent catalog of these patterns online. I highly recommend that you check them out.

Django also implements a number of these patterns. The following table lists a few of them:

Fowler pattern

Django component

Explanation

Active record

Django models

Encapsulate the database access and add domain logic on that data

Class table inheritance

Model inheritance

Each entity in the hierarchy is mapped to a separate table

Identity field

ID field

Saves a database ID field in an object to maintain identity

Template view

Django templates

Render into HTML by embedding markers in HTML

Are there more patterns?

Yes, of course. Patterns are discovered all the time. Like living beings, some mutate and form new patterns, for instance, MVC variants such as Model-view-presenter (MVP), Hierarchical model-view-controller (HMVC), or Model View ViewModel (MVVM).

Patterns also evolve with time, as better solutions to known problems are identified. For example, Singleton pattern was once considered to be a design pattern but now is considered to be an anti-pattern due to the shared state it introduces, similar to using global variables. An anti-pattern can be defined as a commonly reinvented but a bad solution to a problem. Some of the other well-known books that catalog patterns are Pattern-oriented software architecture (POSA) by Buschmann, Meunier, Rohnert, Sommerlad, and Sta; Enterprise Integration Patterns by Hohpe and Woolf; and The Design of Sites: Patterns, Principles, and Processes for Crafting a Customer-Centered Web Experience by Duyne, Landay, and Hong.

 

Patterns in this book


This book will cover Django-specific design and architecture patterns, which would be useful to a Django developer. This is how each pattern will be presented:

Pattern name

The heading is the pattern name. If it is a well-known pattern, the commonly used name is used; otherwise, a terse, self-descriptive name has been chosen. Names are important, as they help in building the pattern vocabulary. All patterns will have the following parts:

  • Problem: This briefly mentions the problem
  • Solution: This summarizes the proposed solution(s)
  • Problem Details: This elaborates the context of the problem and possibly gives an example
  • Solution Details: This explains the solution(s) in general terms and provides a sample Django implementation

Criticism of patterns

Despite their near universal usage, patterns have their share of criticism too. The most common arguments against them are as follows:

  • Patterns compensate for the missing language features: Peter Norvig found that 16 of the 23 patterns in design patterns were invisible or simpler in dynamic languages such as Lisp or Python. For instance, as functions are already objects in Python, it would be unnecessary to create separate classes to implement strategy patterns.
  • Patterns repeat best practices: Many patterns are essentially formalizations of best practices, such as separation of concerns, and could seem redundant.
  • Patterns can lead to over-engineering: Implementing the pattern might be less efficient and excessive compared to a simpler solution.

How to use patterns

Although some of the previous criticisms are quite valid, they are based on how patterns are misused. Here is some advice that can help you understand how best to use design patterns:

  • Patterns are best used to communicate that you are following a well-understood design approach
  • Don't implement a pattern if your language supports a direct solution
  • Don't try to retrofit everything in terms of patterns
  • Use a pattern only if it is the most elegant solution in your context
  • Don't be afraid to create new patterns

Python Zen and Django's design philosophy

Generally, the Python community uses the term Pythonic to describe a piece of idiomatic code. It typically refers to the principles laid out in The Zen of Python. Written like a poem, it is extremely useful to describe such a vague concept.

Note

Try entering import this in a Python prompt to view The Zen of Python.

Furthermore, Django developers have crisply documented their design philosophies while designing the framework at https://docs.djangoproject.com/en/dev/misc/design-philosophies/.

While the document describes the thought process behind how Django was designed, it is also useful for developers using Django to build applications. Certain principles such as Don't Repeat Yourself (DRY), loose coupling, and tight cohesion can help you write more maintainable and idiomatic Django applications.

Django or Python best practices suggested by this book would be formatted in the following manner:

Note

Use BASE_DIR in settings.py and avoid hardcoding directory names.

 

Summary


In this chapter, we looked at why people choose Django over other web frameworks, its interesting history, and how it works. We also examined design patterns, popular pattern collections, and best practices.

In the next chapter, we will take a look at the first few steps in the beginning of a Django project, such as gathering requirements, creating mockups, and setting up the project.

About the Author

  • Arun Ravindran

    Arun Ravindran is an avid speaker and blogger who has been tinkering with Django since 2007 for projects ranging from intranet applications to social networks. He is a long-time open source enthusiast and Python developer. His articles and screencasts have been invaluable to the rapidly growing Django community. He is currently a developer member of the Django Software Foundation. Arun is also a movie buff and loves graphic novels and comics.

    Browse publications by this author

Latest Reviews

(8 reviews total)
Provides a good end to end explanation of Django that many other books fail to do.
Velmi přínosné a dobře se čte..
Very good content, with sufficient context. Much more than a sum of patterns.

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