The Django Book

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Chapter 1: Introduction to Django

This book is about Django, a Web development framework that saves you time and makes Web development a joy. Using Django, you can build and maintain high-quality Web applications with minimal fuss.

At its best, Web development is an exciting, creative act; at its worst, it can be a repetitive, frustrating nuisance. Django lets you focus on the fun stuff the crux of your Web application while easing the pain of the repetitive bits. In doing so, it provides high-level abstractions of common Web development patterns, shortcuts for frequent programming tasks, and clear conventions for how to solve problems. At the same time, Django tries to stay out of your way, letting you work outside the scope of the framework as needed.

The goal of this book is to make you a Django expert. The focus is twofold. First, we explain, in depth, what Django does and how to build Web applications with it. Second, we discuss higher-level concepts where appropriate, answering the question How can I apply these tools effectively in my own projects? By reading this book, youll learn the skills needed to develop powerful Web sites quickly, with code that is clean and easy to maintain.

In this chapter, we provide a high-level overview of Django.

What Is a Web Framework?

Django is a prominent member of a new generation of Web frameworks . So what exactly does that term mean?

To answer that question, lets consider the design of a Web application written using the Common Gateway Interface (CGI) standard, a popular way to write Web applications circa 1998. In those days, when you wrote a CGI application, you did everything yourself the equivalent of baking a cake from scratch. For example, heres a simple CGI script, written in Python, that displays the ten most recently published books from a database:

#!/usr/bin/python

import MySQLdb

print "Content-Type: text/html"
print
print "<html><head><title>Books</title></head>"
print "<body>"
print "<h1>Books</h1>"
print "<ul>"

connection = MySQLdb.connect(user='me', passwd='letmein', db='my_db')
cursor = connection.cursor()
cursor.execute("SELECT name FROM books ORDER BY pub_date DESC LIMIT 10")
for row in cursor.fetchall():
    print "<li>%s</li>" % row[0]

print "</ul>"
print "</body></html>"

connection.close()

This code is straightforward. First, it prints a Content-Type line, followed by a blank line, as required by CGI. It prints some introductory HTML, connects to a database and executes a query that retrieves the latest ten books. Looping over those books, it generates an HTML unordered list. Finally, it prints the closing HTML and closes the database connection.

With a one-off dynamic page such as this one, the write-it-from-scratch approach isnt necessarily bad. For one thing, this code is simple to comprehend even a novice developer can read these 16 lines of Python and understand all it does, from start to finish. Theres nothing else to learn; no other code to read. Its also simple to deploy: just save this code in a file called latestbooks.cgi , upload that file to a Web server, and visit that page with a browser.

But as a Web application grows beyond the trivial, this approach breaks down, and you face a number of problems:

What happens when multiple pages need to connect to the database? Surely that database-connecting code shouldnt be duplicated in each individual CGI script, so the pragmatic thing to do would be to refactor it into a shared function.

Should a developer really have to worry about printing the Content-Type line and remembering to close the database connection? This sort of boilerplate reduces programmer productivity and introduces opportunities for mistakes. These setup- and teardown-related tasks would best be handled by some common infrastructure.

What happens when this code is reused in multiple environments, each with a separate database and password? At this point, some environment-specific configuration becomes essential.

What happens when a Web designer who has no experience coding Python wishes to redesign the page? Ideally, the logic of the page the retrieval of books from the database would be separate from the HTML display of the page, so that a designer could edit the latter without affecting the former.

These problems are precisely what a Web framework intends to solve. A Web framework provides a programming infrastructure for your applications, so that you can focus on writing clean, maintainable code without having to reinvent the wheel. In a nutshell, thats what Django does.

The MVC Design Pattern

Lets dive in with a quick example that demonstrates the difference between the previous approach and that undertaken using a Web framework. Heres how you might write the previous CGI code using Django:

# models.py (the database tables)

from django.db import models

class Book(models.Model):
    name = models.CharField(maxlength=50)
    pub_date = models.DateField()


# views.py (the business logic)

from django.shortcuts import render_to_response
from models import Book

def latest_books(request):
    book_list = Book.objects.order_by('-pub_date')[:10]
    return render_to_response('latest_books.html', {'book_list': book_list})


# urls.py (the URL configuration)

from django.conf.urls.defaults import *
import views

urlpatterns = patterns('',
    (r'latest/$', views.latest_books),
)


# latest_books.html (the template)

<html><head><title>Books</title></head>
<body>
<h1>Books</h1>
<ul>
{% for book in book_list %}
<li>{{ book.name }}</li>
{% endfor %}
</ul>
</body></html>

Dont worry about the particulars of how this works just yet we just want you to get a feel for the overall design. The main thing to note here is the separation of concerns :

The models.py file contains a description of the database table, as a Python class. This is called a model . Using this class, you can create, retrieve, update, and delete records in your database using simple Python code rather than writing repetitive SQL statements.

The views.py file contains the business logic for the page, in the latest_books() function. This function is called a view .

The urls.py file specifies which view is called for a given URL pattern. In this case, the URL /latest/ will be handled by the latest_books() function.

The latest_books.html is an HTML template that describes the design of the page.

Taken together, these pieces loosely follow the Model-View-Controller (MVC) design pattern. Simply put, MVC defines a way of developing software so that the code for defining and accessing data (the model) is separate from request routing logic (the controller), which in turn is separate from the user interface (the view).

A key advantage of such an approach is that components are loosely coupled . That is, each distinct piece of a Django-powered Web application has a single key purpose and can be changed independently without affecting the other pieces. For example, a developer can change the URL for a given part of the application without affecting the underlying implementation. A designer can change a pages HTML without having to touch the Python code that renders it. A database administrator can rename a database table and specify the change in a single place, rather than having to search and replace through a dozen files.

In this book, each component of this stack gets its own chapter. For example, Chapter 3 covers views, Chapter 4 covers templates, and Chapter 5 covers models. Chapter 5 also discusses Djangos MVC philosophies in depth.

Djangos History

Before we dive into more code, we should take a moment to explain Djangos history. Its helpful to understand why the framework was created, because a knowledge of the history will put into context why Django works the way it does.

If youve been building Web applications for a while, youre probably familiar with the problems in the CGI example we presented earlier. The classic Web developers path goes something like this:

Write a Web application from scratch.

Write another Web application from scratch.

Realize the application from step 1 shares much in common with the application from step 2.

Refactor the code so that application 1 shares code with application 2.

Repeat steps 2-4 several times.

Realize youve invented a framework.

This is precisely how Django itself was created!

Django grew organically from real-world applications written by a Web development team in Lawrence, Kansas. It was born in the fall of 2003, when the Web programmers at the Lawrence Journal-World newspaper, Adrian Holovaty and Simon Willison, began using Python to build applications. The World Online team, responsible for the production and maintenance of several local news sites, thrived in a development environment dictated by journalism deadlines. For the sites including LJWorld.com, Lawrence.com, and KUsports.com journalists (and management) demanded that features be added and entire applications be built on an intensely fast schedule, often with only days or hours notice. Thus, Adrian and Simon developed a time-saving Web development framework out of necessity it was the only way they could build maintainable applications under the extreme deadlines.

In summer 2005, after having developed this framework to a point where it was efficiently powering most of World Onlines sites, the World Online team, which now included Jacob Kaplan-Moss, decided to release the framework as open source software. They released it in July 2005 and named it Django, after the jazz guitarist Django Reinhardt.

Although Django is now an open source project with contributors across the planet, the original World Online developers still provide central guidance for the frameworks growth, and World Online contributes other important aspects such as employee time, marketing materials, and hosting/bandwidth for the frameworks Web site (http://www.djangoproject.com/).

This history is relevant because it helps explain two key matters. The first is Djangos sweet spot. Because Django was born in a news environment, it offers several features (particularly its admin interface, covered in Chapter 6) that are particularly well suited for content sites sites like eBay, craigslist.org, and washingtonpost.com that offer dynamic, database-driven information. (Dont let that turn you off, though although Django is particularly good for developing those sorts of sites, that doesnt preclude it from being an effective tool for building any sort of dynamic Web site. Theres a difference between being particularly effective at something and being ineffective at other things.)

The second matter to note is how Djangos origins have shaped the culture of its open source community. Because Django was extracted from real-world code, rather than being an academic exercise or commercial product, it is acutely focused on solving Web development problems that Djangos developers themselves have faced and continue to face. As a result, Django itself is actively improved on an almost daily basis. The frameworks developers have a keen interest in making sure Django saves developers time, produces applications that are easy to maintain, and performs well under load. If nothing else, the developers are motivated by their own selfish desires to save themselves time and enjoy their jobs. (To put it bluntly, they eat their own dog food.)

How to Read This Book

In writing this book, we tried to strike a balance between readability and reference, with a bias toward readability. Our goal with this book, as stated earlier, is to make you a Django expert, and we believe the best way to teach is through prose and plenty of examples, rather than a providing an exhaustive but bland catalog of Django features. (As someone once said, you cant expect to teach somebody how to speak merely by teaching them the alphabet.)

With that in mind, we recommend that you read Chapters 1 through 7 in order. They form the foundation of how to use Django; once youve read them, youll be able to build Django-powered Web sites. The remaining chapters, which focus on specific Django features, can be read in any order.

The appendixes are for reference. They, along with the free documentation at http://www.djangoproject.com/, are probably what youll flip back to occasionally to recall syntax or find quick synopses of what certain parts of Django do.

Required Programming Knowledge

Readers of this book should understand the basics of procedural and object-oriented programming: control structures (if , while , and for ), data structures (lists, hashes/dictionaries), variables, classes, and objects.

Experience in Web development is, as you may expect, very helpful, but its not required to read this book. Throughout the book, we try to promote best practices in Web development for readers who lack this type of experience.

Required Python Knowledge

At its core, Django is simply a collection of libraries written in the Python programming language. To develop a site using Django, you write Python code that uses these libraries. Learning Django, then, is a matter of learning how to program in Python and understanding how the Django libraries work.

If you have experience programming in Python, you should have no trouble diving in. By and large, the Django code doesnt perform black magic (i.e., programming trickery whose implementation is difficult to explain or understand). For you, learning Django will be a matter of learning Djangos conventions and APIs.

If you dont have experience programming in Python, youre in for a treat. Its easy to learn and a joy to use! Although this book doesnt include a full Python tutorial, it highlights Python features and functionality where appropriate, particularly when code doesnt immediately make sense. Still, we recommend you read the official Python tutorial, available online at http://docs.python.org/tut/. We also recommend Mark Pilgrims free book Dive Into Python , available at http://www.diveintopython.org/ and published in print by Apress.

New Django Features

As we noted earlier, Django is frequently improved, and it will likely have a number of useful even essential new features by the time this book is published. Thus, our goal as authors of this book is twofold:

Make sure this book is as future-proof as possible, so that whatever you read here will still be relevant in future Django versions

Actively update this book on its Web site, http://www.djangobook.com/, so you can access the latest and greatest documentation as soon as we write it

If you want to implement something with Django that isnt explained in this book, check the latest version of this book on the aforementioned Web site, and also check the official Django documentation.

Getting Help

One of the greatest benefits of Django is its kind and helpful user community. For help with any aspect of Django from installation, to application design, to database design, to deployment feel free to ask questions online.

The django-users mailing list is where thousands of Django users hang out to ask and answer questions. Sign up for free at http://www.djangoproject.com/r/django-users.

The Django IRC channel is where Django users hang out to chat and help each other in real time. Join the fun by logging on to #django on the Freenode IRC network.