Boston Python Workshop/Saturday/Web app project

Overview

On Saturday, you can write and deploy a web application. It's an online poll where visitors can view choices (a bit of text, plus an image) and vote the option up and down.

My notes about this

• Based on http://www.wiki.devchix.com/index.php?title=Rails_3_Curriculum and http://docs.djangoproject.com/en/dev/intro/tutorial01/

Overview

Note: This is one long page. It will take most of the afternoon to go through it.

If you stick with it, you will have deployed a web application to the world, where other people can play with it, and where you can modify it.

This is based heavily on the official tutorial for the Django web programming framework.

Writing your first Django app, part 1

Let’s learn by example.

Throughout this tutorial, we’ll walk you through the creation of a basic poll application.

It’ll consist of two parts:

• A public site that lets people view polls and vote in them.
• An admin site that lets you add, change and delete polls.

Switch to the right directory

• In a terminal (or GitBash), get into the django_projects directory we created in the Friday setup portion of the tutorial. You can do that by typing this into your terminal:

cd Desktop
cd django_projects

In the Friday setup portion of the workshop, you already saw how to use the django-admin.py command to start a project. The workshop coordinators already created a project, and you already forked it on Github. So now, you'll clone that to your computer.

• Go to http://github.com/
• Find your clone of workshop_mysite. Find the SSH URL for it, and copy that to the clipboard.
• In the terminal, type: git clone followed by the URL for your personal fork of the workshop_mysite repository.
• Make sure you can "cd" into it:

cd workshop_mysite

Look at the files

Let’s look at files are in the project:

workshop_mysite/
   __init__.py
   manage.py
   settings.py
   urls.py

These files are:

• __init__.py: An empty file that tells Python that this directory should be considered a Python package. (Read more about packages in the official Python docs if you're a Python beginner.)
• manage.py: A command-line utility that lets you interact with this Django project in various ways. You can read all the details about manage.py in django-admin.py and manage.py.
• settings.py: Settings/configuration for this Django project. Django settings will tell you all about how settings work.
• urls.py: The URL declarations for this Django project; a "table of contents" of your Django-powered site. You can read more about URLs in URL dispatcher.

The development server

Let's verify this worked. If you haven't already, and run the command python manage.py runserver. You'll see the following output on the command line:

Validating models... 0 errors found.

Django version 1.0, using settings 'mysite.settings' Development server is running at http://127.0.0.1:8000/ Quit the server with CONTROL-C.

You've started the Django development server, a lightweight Web server written purely in Python. The Django maintainers include this web server, but on a "deployment" like alwaysdata.com, you typically tie Django into an existing server like Apache.

Now that the server's running, visit http://127.0.0.1:8000/ with your Web browser. You'll see a "Welcome to Django" page, in pleasant, light-blue pastel. It worked!

Fixing security settings

Right now, everyone in the workshop has the same SECRET_KEY. According to the Django documentation, that is bad. So open up settings.py in your editor (for example, Komodo Edit).

settings.py is a Python script that only contains variable definitions. (Django looks at the values of these variables when it runs your web app.)

Find the variable named SECRET_KEY and set it to whatever string you want. Go on, we'll wait.

Database setup

Keep looking at settings.py: it has a dictionary with one key: default.

The value is itself another dictionary with information about the site's default database. You can see from the NAME that the Django project uses a file called database.db to store information.

Pop quiz: Does database.db exist right now?

While you're editing settings.py, take note of the INSTALLED_APPS setting towards the bottom of the file. That variable holds the names of all Django applications that are activated in this Django instance. Apps can be used in multiple projects, and you can package and distribute them for use by others in their projects.

By default, INSTALLED_APPS contains the following apps, all of which come with Django:

• django.contrib.auth -- An authentication system.
• django.contrib.contenttypes -- A framework for content types.
• django.contrib.sessions -- A session framework.
• django.contrib.sites -- A framework for managing multiple sites with one Django installation.
• django.contrib.messages -- A messaging framework.

These applications are included by default as a convenience.

Each of these applications makes use of at least one database table, so we need to create the tables in the database before we can use them. To do that, run the following command:

python manage.py syncdb

The syncdb command looks at the INSTALLED_APPS setting and creates any necessary database tables according to the database settings in your settings.py file. You'll see a message for each database table it creates, and you'll get a prompt asking you if you'd like to create a superuser account for the authentication system. Go ahead and do that.

Part 1.5: Creating polls

Creating models

Now that your environment -- a "project" -- is set up, you're set to start building the poll.

Each application you write in Django consists of a Python package, somewhere on your Python path, that follows a certain convention. Django comes with a utility that automatically generates the basic directory structure of an app, so you can focus on writing code rather than creating directories.

Projects vs. apps

We've talked a little about Django apps and projects. You might be wondering what the difference is.

Here are the things to know:

• A project contains one more apps.
• An app is component of a website that does something. For example, the Django administration app is something you'll see later in this tutorial
• A project corresponds to a website: it contains a settings.py file, so it has a corresponding database.

Django apps can live anywhere on the "Python path." That just means that you have to be able to import them when your Django project runs.

In this tutorial, we'll create our poll app in the workshop_mysite directory for simplicity. In the future, when you decide that the world needs to be able to use your poll app and plug it into their own projects, you can publish that directory separately.

To create your app, make sure you're in the workshop_mysite directory and type this command:

python manage.py startapp polls

That'll create a directory polls, which is laid out like this:

polls/
   __init__.py
   models.py
   tests.py
   views.py

This directory structure will house the poll application.

The first step in writing a database Web app in Django is to define your models -- essentially, your database layout, with additional metadata.

Philosophy

A model is the single, definitive source of data about your data. It contains the essential fields and behaviors of the data you're storing. Django follows the DRY ("Don't Repeat Yourself") Principle. The goal is to define your data model in one place and automatically derive things from it.

(If you've used SQL before, you might be interested to know that each Django model corresponds to a SQL table.)

In our simple poll app, we'll create two models: polls and choices. A poll has a question and a publication date. A choice has two fields: the text of the choice and a vote tally. Each choice is associated with a poll. (FIXME: Add image to Choice.)

These concepts are represented by simple Python classes. Edit the polls/models.py file so it looks like this:

from django.db import models

class Poll(models.Model):
    question = models.CharField(max_length=200)
    pub_date = models.DateTimeField('date published')

class Choice(models.Model):
    poll = models.ForeignKey(Poll)
    choice = models.CharField(max_length=200)
    votes = models.IntegerField()

All models in Django code are represented by a class that subclasses django.db.models.Model. Each model has a number of class variables, each of which represents a database field in the model.

Each field is represented by an instance of a Field class -- e.g., CharField for character fields and DateTimeField for datetimes. This tells Django what type of data each field holds.

The name of each Field instance (e.g. question or pub_date ) is the field's name, in machine-friendly format. You'll use this value in your Python code, and your database will use it as the column name.

The pub_date field has something unique about it: a human-readable name, "date published". One feature of Django Field classes is that if you pass in a first argument for most fields, Django will use this in a couple of introspective parts of Django, and it doubles as documentation. If the human-readable isn't provided, Django will use the machine-readable name. In this example, we've only defined a human-readable name for Poll.pub_date. For all other fields in this model, the field's machine-readable name will suffice as its human-readable name.

Some Field classes have required elements. CharField, for example, requires that you give it a max_length. That's used not only in the database schema, but in validation, as we'll soon see.

Finally, note a relationship is defined, using ForeignKey. That tells Django each Choice is related to a single Poll. Django supports all the common database relationships: many-to-ones, many-to-manys and one-to-ones.

Activating models

That small bit of model code gives Django a lot of information. With it, Django is able to:

• Create a database schema (CREATE TABLE statements) for this app.
• Create a Python database-access API for accessing Poll and Choice objects.

But first we need to tell our project that the polls app is installed.

Django Philosophy

Django apps are "pluggable": You can use an app in multiple projects, and you can distribute apps, because they don't have to be tied to a given Django installation.

Edit the settings.py file again, and change the INSTALLED_APPS setting to include the string 'polls'. So it'll look like this:

INSTALLED_APPS = (
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.sites',
    'polls',
)

Now Django knows to include the polls app.

If you care about SQL, you can try the following command:

• python manage.py sql polls

For now, let's just Django's syncdb tool to create the database tables for Poll objects:

python manage.py syncdb

The syncdb looks for apps that have not yet been set up. To set them up, it runs the necessary SQL commands against your database. This creates all the tables, initial data and indexes for any apps you have added to your project since the last time you ran syncdb. syncdb can be called as often as you like, and it will only ever create the tables that don't exist.

Read the django-admin.py documentation for full information on what the manage.py utility can do.

Playing with the database

During Friday setup, you installed SQLite Manager into your system's Firefox. Now's a good time to open it up.

• FIXME

Playing with the API

Now, let's hop into the interactive Python shell and play around with the free API Django gives you. To invoke the Python shell, use this command:

python manage.py shell

We're using this instead of simply typing "python", because manage.py sets up the project's environment for you. "Setting up the environment" involves two things:

1. Making sure polls is on the right path to be imported.
2. Setting the DJANGO_SETTINGS_MODULE environment variable, which gives Django the path to your settings.py file.

Once you're in the shell, explore the database API:

Let's import the model classes we just wrote:

>>> from polls.models import Poll, Choice

To list all the current Polls:

>>> Poll.objects.all()
[]

It is an empty list because there are no polls. Let's add one!

>>> import datetime
>>> p = Poll(question="What's up?", pub_date=datetime.datetime.now())

Then we'll save the object into the database. You have to call save() explicitly.

>>> p.save()

Great. Now, because it's been saved, it has an ID in the database. You can see that by typing this into the Python shell:

>>> p.id
1

You can also access the database columns (Fields, in Django parlance) as Python attributes:

>>> p.question
"What's up?"
>>> p.pub_date
datetime.datetime(2007, 7, 15, 12, 00, 53)

We can time travel back in time! Or at least, we can send the Poll back in time:

# Change values by changing the attributes, then calling save().
>>> p.pub_date = datetime.datetime(2007, 4, 1, 0, 0)
>>> p.save()

Finally, we can also ask Django to show a list of all the Poll objects available:

>>> Poll.objects.all()
[<Poll: Poll object>]

Wait a minute. <Poll: Poll object> is, utterly, an unhelpful representation of this object. Let's fix that by editing the polls model (in the polls/models.py file) and adding a __unicode__() method to both Poll and Choice:

class Poll(models.Model):
    # ...
    def __unicode__(self):
        return self.question

class Choice(models.Model):
    # ...
    def __unicode__(self):
        return self.choice

It's important to add __unicode__() methods to your models, not only for your own sanity when dealing with the interactive prompt, but also because objects' representations are used throughout Django's automatically-generated admin.

(If you're using to Python programming from a time in the past, you might have seen __str__(). Django prefers you use __unicode__() instead.)

Note these are normal Python methods. Let's add a custom method, just for demonstration:

import datetime
# ...
class Poll(models.Model):
    # ...
    def was_published_today(self):
        return self.pub_date.date() == datetime.date.today()

Note the addition of import datetime to reference Python's standard datetime module.

Save these changes and start a new Python interactive shell by running python manage.py shell again:

>>> from polls.models import Poll, Choice

Check it out: our __unicode__() addition worked:

>>> Poll.objects.all()
[<Poll: What's up?>]

If you want to search your database, you can do it using the filter method on the objects attribute of Poll. For example:

>>> p = Poll.objects.filter(question="What's up?")
>>> p
[<Poll: What's up?>]
>>> p.id
1

If you try to search for a poll that does not exist, filter will give you the empty list. The get method will always return one hit, or raise an exception.

>>> Poll.objects.filter(question="What time is it?")
[]

>>> Poll.objects.get(id=1)
<Poll: What's up?>
>>> Poll.objects.get(id=2)
Traceback (most recent call last):
    ...
DoesNotExist: Poll matching query does not exist.

Adding choices

Right now, we have a Poll in the database, but it has no Choices. See:

>>> p = Poll.objects.get(pk=1)
>>> p.choice_set.all()
[]

So let's create three choices:

>>> p.choice_set.create(choice='Not much', votes=0)
<Choice: Not much>
>>> p.choice_set.create(choice='The sky', votes=0)
<Choice: The sky>
>>> c = p.choice_set.create(choice='Just hacking again', votes=0)
<Choice: Just hacking again>

Every Choice can find the Poll that it belongs to:

>>> c.poll
<Poll: What's up?>

We just used this, but now I'll explain it: Because a Poll can have more than one Choice, Django creates the choice_set attribute on each Poll. You can use that to look at the list of available Choices, or to create them.

>>> p.choice_set.all()
[<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]
>>> p.choice_set.count()
3

Visualize the database in SQLite Manager

• FIXME

Enough databases for now

In the next section of the tutorial, you'll write views that let other people look at your polls.

Part 2: Letting the world see your polls, with views

We have all these polls in our database. However, no one can see them, because we never made any web pages that render the polls into HTML.

Let's change that with Django views.

Philosophy

A view is a “type” of Web page in your Django application that generally serves a specific function and has a specific template. For example, in a Weblog application, you might have the following views:

• Blog homepage – displays the latest few entries.
• Entry “detail” page – permalink page for a single entry.
• Year-based archive page – displays all months with entries in the given year.
• Month-based archive page – displays all days with entries in the given month.
• Day-based archive page – displays all entries in the given day.
• Comment action – handles posting comments to a given entry.

In our poll application, we’ll have the following four views:

• Poll “index” page – displays the latest few polls.
• Poll “detail” page – displays a poll question, with no results but with a form to vote.
• Poll “results” page – displays results for a particular poll.
• Vote action – handles voting for a particular choice in a particular poll.

In Django, each view is represented by a simple Python function.

Design your URLs

The first step of writing views is to design your URL structure. You do this by creating a Python module, called a URLconf. URLconfs are how Django associates a given URL with given Python code.

When a user requests a Django-powered page, the system looks at the ROOT_URLCONF setting, which contains a string in Python dotted syntax. Django loads that module and looks for a module-level variable called urlpatterns, which is a sequence of tuples in the following format:

(regular expression, Python callback function [, optional dictionary])

Django starts at the first regular expression and makes its way down the list, comparing the requested URL against each regular expression until it finds one that matches.

You might ask, "What's a regular expression?" Regular expressions are patterns for matching text. In this case, we're matching the URLs people go to, and using regular expressions to categorize them into different kinds of

(If (like me) you think regular expressions are intriguing and thrilling, you can read the Dive into Python guide to regular expressions sometime. Or you can look at this comic.)

In addition to matching text, regular expressions can capture text: regexps use parentheses to wrap the parts they're capturing.

For Django, when a regular expression matches the URL that a web surfer requests, Django extracts the captured values and passes them to a function of your choosing. This is the role of the callback function above.

Adding URLs to urls.py

When we ran django-admin.py startproject workshop_mysite to create the project, Django created a default URLconf. Take a look at settings.py for this line:

ROOT_URLCONF = 'workshop_mysite.urls'

That means that the default URLconf in workshop_mysite/urls.py.

Time for an example. Edit mysite/urls.py so it looks like this:

from django.conf.urls.defaults import *

urlpatterns = patterns(,
    (r'^polls/$', 'polls.views.index'),
    (r'^polls/(\d+)/$', 'polls.views.detail'),
    (r'^polls/(\d+)/results/$', 'polls.views.results'),
    (r'^polls/(\d+)/vote/$', 'polls.views.vote'),
)

This is worth a review. When somebody requests a page from your Web site -- say, "/polls/23/", Django will load the urls.py Python module, because it's pointed to by the ROOT_URLCONF setting. It finds the variable named urlpatterns and traverses the regular expressions in order. When it finds a regular expression that matches -- r'^polls/(\d+)/$' -- it loads the function detail() from polls/views.py. Finally, it calls that detail() function like so:

detail(request=<HttpRequest object>, '23')

The '23' part comes from (\d+). Using parentheses around a pattern "captures" the text matched by that pattern and sends it as an argument to the view function; the \d+ is a regular expression to match a sequence of digits (i.e., a number).

(In Django, you have total control over the way your URLs look. People on the web won't see cruft like .py or .php at the end of your URLs.)

Finally: Write your first view

Well, we haven't created any views yet -- we just have the URLconf. But let's make sure Django is following the URLconf properly.

Fire up the Django development Web server:

python manage.py runserver

Now go to "http://localhost:8000/polls/" in your Web browser. You should get a pleasantly-colored error page with the following message:

ViewDoesNotExist at /polls/

Tried index in module polls.views. Error was: 'module'
object has no attribute 'index'

This error happened because you haven't written a function index() in the module polls/views.py.

Try "/polls/23/", "/polls/23/results/" and "/polls/23/vote/". The error messages tell you which view Django tried (and failed to find, because you haven't written any views yet).

Time to write the first view. Open the file polls/views.py and put the following Python code in it:

from django.http import HttpResponse

def index(request):
    return HttpResponse("Hello, world. You're at the poll index.")

This is the simplest view possible. Go to "/polls/" in your browser, and you should see your text.

Now lets add a few more views. These views are slightly different, because they take an argument (which, remember, is passed in from whatever was captured by the regular expression in the URLconf):

def detail(request, poll_id):
    return HttpResponse("You're looking at poll %s." % poll_id)

def results(request, poll_id):
    return HttpResponse("You're looking at the results of poll %s." % poll_id)

def vote(request, poll_id):
    return HttpResponse("You're voting on poll %s." % poll_id)

Take a look in your browser, at "/polls/34/". It'll run the detail() method and display whatever ID you provide in the URL. Try "/polls/34/results/" and "/polls/34/vote/" too -- these will display the placeholder results and voting pages.

Write views that actually do something

Each view is responsible for doing one of two things: Returning an HttpResponse object containing the content for the requested page, or raising an exception such as Http404. The rest is up to you.

Your view can read records from a database, or not. It can use a template system such as Django's -- or not. It can generate a PDF file, output XML, create a ZIP file on the fly, anything you want, using whatever Python libraries you want.

All Django wants is that HttpResponse. Or an exception.

Most of the Django views in the world use Django's own database API, which we covered in Tutorial 1. Let's do that, too. Here's one stab at the index() view, which displays the latest 5 poll questions in the system, separated by commas, according to publication date:

from polls.models import Poll
from django.http import HttpResponse

def index(request):
    latest_poll_list = Poll.objects.all().order_by('-pub_date')[:5]
    output = ', '.join([p.question for p in latest_poll_list])
    return HttpResponse(output)

There's a problem here, though: The page's design is hard-coded in the view. If you want to change the way the page looks, you'll have to edit this Python code. So let's use Django's template system to separate the design from Python:

from django.shortcuts import render_to_response
from polls.models import Poll

def index(request):
    latest_poll_list = Poll.objects.all().order_by('-pub_date')[:5]
    context = {'latest_poll_list': latest_poll_list}
    return render_to_response('polls/index.html', context)

To recap what this does:

• Creates a variable called latest_poll_list. Django queries the database for all Poll objects, ordered by pub_date with most recent first, and uses slicing to get the first five.
• Creates a variable called context that is a dictionary with one key.
• Evaluates the render_to_response function with two arguments, and returns whatever that returns.

render_to_response loads the template called "polls/index.html" and passes it a value as context. The context is a dictionary mapping template variable names to Python objects.

If you can read this this view function without being overwhelmed, then you understand the basics of Django views. Now is a good time to reflect and make sure you do. (If you have questions, ask a volunteer for help.)

Reload the page. Now you'll see an error:

TemplateDoesNotExist at /polls/
polls/index.html

Ah. There's no template yet. Let's make one.

First, let's make a directory where templates will live. We'll need a templates directory right alongside the views.py for the polls app. This is what I would do:

mkdir -p polls/templates/polls

Then create a file in there called index.html. Within that, create a file called index.html. Note that our loader.get_template('polls/index.html') code from above maps to "[template_directory]/polls/index.html" on the filesystem.

Put the following code in that template:

{% if latest_poll_list %}

{% for poll in latest_poll_list %}
• <a href="/polls/Template:Poll.id/">Template:Poll.question</a>
{% endfor %}

{% else %}

No polls are available.

{% endif %}

Load the page in your Web browser, and you should see a bulleted-list containing the "What's up" poll from Tutorial 1. The link points to the poll's detail page.

Raising 404

Now, let's tackle the poll detail view -- the page that displays the question for a given poll. This view uses Python exceptions:

from django.http import Http404
# ...
def detail(request, poll_id):
    try:
        p = Poll.objects.get(pk=poll_id)
    except Poll.DoesNotExist:
        raise Http404
    return render_to_response('polls/detail.html', {'poll': p})

The new concept here: The view raises the Http404 exception if a poll with the requested ID doesn't exist.

If you'd like to quickly get the above example working, just:

Template:Poll

will get you started for now.

Add it to a new template file that you create, detail.html.

Does your detail view work? Try it: http://127.0.0.1:8000/polls/detail/1/

Adding more detail

Let's give the detail view some more detail.

We pass in a variable called poll that points to an instance of the Poll class. So you can pull out more information by writing this into the "polls/detail.html" template:

Template:Poll.question

{% for choice in poll.choice_set.all %}
• Template:Choice.choice
{% endfor %}

The template system uses dot-lookup syntax to access variable attributes. Django's template language is a bit sloppy: in pure Python, the . (dot) only lets you get attributes from objects. In this example, we are just doing attribute lookup, but in general if you're not sure how to get data out of an object in Django, try dot.

Method-calling happens in the {% for %} loop: poll.choice_set.all is interpreted as the Python code poll.choice_set.all(), which returns a sequence of Choice objects and is suitable for use in the {% for %} tag.

Adding some style

The web page looks okay, but it is somewhat drab.

FIXME: CSS