Boston Python Workshop/Saturday/Web app project

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[edit] 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.

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.

This page should say what you should actually expect to know. It is okay that you don't understand everything you are typing in. After a lot more learning, you will be able to. The first time, though, it's okay if you don't. Will and Katie have feedback for this page.

[edit] Writing your first Django app, part 1

Let’s learn by example.

This tutorial walks 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.

[edit] 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

[edit] Look at the files

Let’s look at files in the project:

workshop_mysite/
   public/
   README.mediawiki
   __init__.py
   manage.py
   settings.py
   urls.py

These files are:

  • README.mediawiki: Many projects come with README files that, well, you should read. This one does, too.
  • public/: This directory contains files the instructors put together so you can easily deploy your web app to Alwaysdata.com.
  • __init__.py: An empty file that tells Python that this directory should be considered a Python module. Because of the __init__.py file, you can use import to import workshop_mysite.
  • 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.

[edit] The development server

Let's verify this worked. Run the command:

python manage.py runserver

You'll see the following output on the command line:

Validating models...
0 errors found.

Django version 1.2, 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!

Exit the server by pressing CONTROL-C on your keyboard.

[edit] 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.

[edit] Database setup

Keep looking at settings.py: The DATABASES variable is 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.

[edit] Part 1.5: Creating polls

[edit] 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.

[edit] 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:

  • 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.

[edit] Django 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 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()

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

Save the models.py file.

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.

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.

[edit] 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.

[edit] 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',
)

Save the settings.py file.

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.

[edit] 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 an utterly unhelpful representation of this object. Let's fix that by editing the polls model Use your text editor to open 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. FIXME: add explanation of why we did this

Save these changes to the models.py file, and then 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:

>>> polls = Poll.objects.filter(question="What's up?")
>>> polls
[<Poll: What's up?>]
>>> polls[0].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.

[edit] Adding choices

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

>>> p = Poll.objects.get(id=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)
>>> c
<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

[edit] Visualize the database in SQLite Manager

When you call .save() on a model instance, Django saves that to the database. (Remember, Django is a web programming framework built around the idea of saving data in a SQL database.)

Where is that database? Take a look at settings.py in your text editor. You can see that database.db is the filename. In settings.py Python calculates the path to the current file.

So now:

  • Open up Firefox
  • Find SQLite Manager in Tools->SQLite Manager
  • In the SQLite Manager menus, choose: Database->Connect Database
  • Find the workshop_mysite/database.db file.

Browse your tables! This is another way of looking at the data you just created.

Note: In order to find the database.db file, you might need to ask SQLite Manager to show you all files, not just the *.sqlite files.

I (the author of this tutorial) think it's really important that you be able to find this database file. So go ahead and do this step. Browse around! Hooray.

When you're satisfied with your Poll data, you can close it.

[edit] Save and share our work

We've done something! Let's share it with the world.

We'll do that with git and Github. On your own computer, get to a Terminal or a GitBash.

Use cd to get into the workshop_mysite directory. If it's a fresh Terminal, this is what you'll do:

cd Desktop
cd django_projects
cd workshop_mysite

Use git add to add the content of your files to git:

git add polls/*.py

And use git commit to commit those files:

git commit -m "I made these files and this is a message describing them"

Finally, use git push to push those up to your Github repository:

git push

Go to your Github account. Find the workshop_mysite repository. Do you see your files?

If so, proceed!

[edit] Enough databases for now

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

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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 is workshop_mysite/urls.py.

Time for an example. Edit the file workshop_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.)

[edit] 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. Save the views.py file, then go to "/polls/" in your browser, and you should see your text.

Now let's add a few more views by adding to the views.py file. 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)

Save the views.py file. Now 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.

[edit] 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. Continue editing the file views.py:

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)

Now go to "http://localhost:8000/polls/" in your Web browser. You should see the text of the first poll. 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

Within that, create a file called index.html.

Put the following code in that template:

 {% if latest_poll_list %}
     <ul>
     {% for poll in latest_poll_list %}
         <li><a href="/polls/{{ poll.id }}/">{{ poll.question }}</a></li>
     {% endfor %}
     </ul>
 {% else %}
     <p>No polls are available.</p>
 {% endif %}

Load the page "http://localhost:8000/polls/" into your Web browser again, 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.

[edit] Raising 404

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

from django.http import Http404
# ...
def detail(request, poll_id):
    try:
        p = Poll.objects.get(id=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 create a new template file and name it detail.html. Enter in it just one line of code:

{{ poll }}

to get you started for now.

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

You can also try to load a poll page that does not exist, just to test out the pretty 404 error: http://127.0.0.1:8000/polls/32/

[edit] 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:

<h1>{{ poll.question }}</h1>
<ul>
{% for choice in poll.choice_set.all %}
    <li>{{ choice.choice }}</li>
{% endfor %}
</ul>

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.

Load the new detail page in your browser: http://127.0.0.1:8000/polls/1/ The poll choices now appear.

[edit] Adding some style

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

FIXME: CSS

[edit] Part 3: Let people vote

[edit] Write a simple form

Let’s update our poll detail template (“polls/detail.html”) from the last tutorial so that the template contains an HTML <form> element:

<h1>{{ poll.question }}</h1>

{% if error_message %}<p><strong>{{ error_message }}</strong></p>{% endif %}

<form action="/polls/{{ poll.id }}/vote/" method="post">
{% csrf_token %}
{% for choice in poll.choice_set.all %}
    <input type="radio" name="choice" value="{{ choice.id }}" />
    <label>{{ choice.choice }}</label><br />
{% endfor %}
<input type="submit" value="Vote" />
</form>

There is a lot going on there. A quick rundown:

  • The above template displays a radio button for each poll choice. The value of each radio button is the associated poll choice's ID. The name of each radio button is "choice". That means, when somebody selects one of the radio buttons and submits the form, the form submission will represent the Python dictionary {'choice': '3'}. That's the basics of HTML forms; you can learn more about them.
  • We set the form's action to
    /polls/{{ poll.id }}/vote/
    , and we set method="post". Normal web pages are requested using GET, but the standards for HTTP indicate that if you are changing data on the server, you must use the POST method. (Whenever you create a form that alters data server-side, use method="post". This tip isn't specific to Django; it's just good Web development practice.)
  • Since we're creating a POST form (which can have the effect of modifying data), we need to worry about Cross Site Request Forgeries. Thankfully, you don't have to worry too hard, because Django comes with a very easy-to-use system for protecting against it. In short, all POST forms that are targeted at internal URLs should use the {% csrf_token %} template tag.

The {% csrf_token %} tag requires information from the request object, which is not normally accessible from within the template context. To fix this, a small adjustment needs to be made to the detail view in the "views.py" file, so that it looks like the following:

from django.template import RequestContext
# ...
def detail(request, poll_id):
    p = get_object_or_404(Poll, pk=poll_id)
    return render_to_response('polls/detail.html', {'poll': p},
                               context_instance=RequestContext(request))

The details of how this works are explained in the documentation for RequestContext.

Now, let's create a Django view that handles the submitted data and does something with it. Remember, in Tutorial 3, we created a URLconf for the polls application that includes this line:

(r'^(?P<poll_id>\d+)/vote/$', 'vote'),

We also created a dummy implementation of the vote() function. Let's create a real version. Add the following to polls/views.py:

from django.shortcuts import get_object_or_404, render_to_response
from django.http import HttpResponseRedirect, HttpResponse
from django.core.urlresolvers import reverse
from django.template import RequestContext
from polls.models import Choice, Poll
# ...
def vote(request, poll_id):
    p = get_object_or_404(Poll, pk=poll_id)
    try:
        selected_choice = p.choice_set.get(pk=request.POST['choice'])
    except (KeyError, Choice.DoesNotExist):
        # Redisplay the poll voting form.
        return render_to_response('polls/detail.html', {
            'poll': p,
            'error_message': "You didn't select a choice.",
        }, context_instance=RequestContext(request))
    else:
        selected_choice.votes += 1
        selected_choice.save()
        # Always return an HttpResponseRedirect after successfully dealing
        # with POST data. This prevents data from being posted twice if a
        # user hits the Back button.
        return HttpResponseRedirect(reverse('polls.views.results', args=(p.id,)))

This code includes a few things we haven't covered yet in this tutorial:

  • request.POST is a dictionary-like object that lets you access submitted data by key name. In this case, request.POST['choice'] returns the ID of the selected choice, as a string. request.POST values are always strings.
  • Note that Django also provides request.GET for accessing GET data in the same way -- but we're explicitly using request.POST in our code, to ensure that data is only altered via a POST call.
  • request.POST['choice'] will raise KeyError if choice wasn't provided in POST data. The above code checks for KeyError and redisplays the poll form with an error message if choice isn't given.
  • After incrementing the choice count, the code returns an HttpResponseRedirect rather than a normal HttpResponse. HttpResponseRedirect takes a single argument: the URL to which the user will be redirected (see the following point for how we construct the URL in this case).

As the Python comment above points out, you should always return an HttpResponseRedirect after successfully dealing with POST data. This tip isn't specific to Django; it's just good Web development practice. That way, if the web surfer hits reload, they get the success page again, rather than re-doing the action.


We are using the reverse() function in the HttpResponseRedirect constructor in this example. This function helps avoid having to hardcode a URL in the view function. It is given the name of the view that we want to pass control to and the variable portion of the URL pattern that points to that view. In this case, using the URLconf we set up in Tutorial 3, this reverse() call will return a string like

     '/polls/3/results/'

... where the 3 is the value of p.id. This redirected URL will then call the 'results' view to display the final page. Note that you need to use the full name of the view here (including the prefix).

After somebody votes in a poll, the vote() view redirects to the results page for the poll. Let's write that view:

def results(request, poll_id):
    p = get_object_or_404(Poll, pk=poll_id)
    return render_to_response('polls/results.html', {'poll': p})

This is almost exactly the same as the detail() view from Tutorial 3. The only difference is the template name. We'll fix this redundancy later.

Now, create a results.html template:

<h1>{{ poll.question }}</h1>

<ul>
{% for choice in poll.choice_set.all %}
    <li>{{ choice.choice }} -- {{ choice.votes }} vote{{ choice.votes|pluralize }}</li>
{% endfor %}
</ul>

<a href="/polls/{{ poll.id }}/">Vote again?</a>

Now, go to /polls/1/ in your browser and vote in the poll. You should see a results page that gets updated each time you vote. If you submit the form without having chosen a choice, you should see the error message.

Does it work?! If so, show your neighbor!

[edit] Part 3.5: Deploy your web app!

You've done a lot of work. It's time to share it with the world.

This workshop follows a workflow very similar to what I personally use in my professional Django projects: using git to store the history of my project on my computer, and using that to synchronize with a web server other people can see.

You've already pushed some work to Github. To get our changes over to alwaysdata. you'll:

  1. Add and commit files on your own computer.
  2. Push your changes to Github.
  3. Connect to your alwaysdata.com account via SSH/PuTTY
  4. Run "git pull" to get the latest version to your Alwaysdata account.

So we'll do those steps in order.

To do the add and commit, open up your Terminal or GitBash:

git add .
git commit -m "More changes"

To push:

git push

Now, open up SSH or PuTTY and connect to your alwaysdata.com account.

Finally, in that terminal:

cd workshop_mysite
git pull

Okay, not quite finally. You might need to go to https://admin.alwaysdata.com/advanced/processes/ and click Restart my applications.

Go to your alwaysdata site's /polls/ page. For me, I'd go to:

You should see your poll!

[edit] Part 4: Editing your polls in the Django admin interface

So far, you've been adding data to your database using the manage.py shell. This is a flexible way to add data, but it has some drawbacks:

  • It's not on the web.
  • A fanatical insistence on precision: You have to write Python code to add data, which means that typos or syntax errors could make your life harder.
  • An unnecessary lack of color.

[edit] Background: Django's built-in admin interface

Generating admin sites for your staff or clients to add, change and delete content is tedious work that doesn’t require much creativity. For that reason, Django entirely automates creation of admin interfaces for models.

Django was written in a newsroom environment, with a very clear separation between “content publishers” and the “public” site. Site managers use the system to add news stories, events, sports scores, etc., and that content is displayed on the public site. Django solves the problem of creating a unified interface for site administrators to edit content.

The admin isn’t necessarily intended to be used by site visitors; it’s for site managers.

[edit] Activate the admin site

The Django admin site is not activated by default – it’s an opt-in thing. To activate the admin site for your installation, do these three things:

  • Open up workshop_mysite/settings.py and add "django.contrib.admin" to your INSTALLED_APPS setting.
  • Run python manage.py syncdb. Since you have added a new application to INSTALLED_APPS, the database tables need to be updated.
  • Edit your workshop_mysite/urls.py file and uncomment the lines that reference the admin – there are three lines in total to uncomment.

[edit] Start the development server

Let’s make sure the development server is running and explore the admin site.

Try going to http://127.0.0.1:8000/admin/. If it does not load, make sure you are still running the development server. You can start the development server like so:

python manage.py runserver

http://127.0.0.1:8000/admin/ should show you the admin site's login screen.

[edit] Enter the admin site

Now, try logging in. (You created a superuser account earlier, when running syncdb for the fist time. If you didn't create one or forgot the password you can create another one.) You should see the Django admin index page.

You should see a few other types of editable content, including groups, users and sites. These are core features Django ships with by default.

[edit] Make the poll app modifiable in the admin

But where's our poll app? It's not displayed on the admin index page.

Just one thing to do: We need to tell the admin that Poll objects have an admin interface. To do this, create a file called admin.py in your polls directory, and edit it to look like this:

from polls.models import Poll
from django.contrib import admin

admin.site.register(Poll)

You'll need to restart the development server to see your changes. Normally, the server auto-reloads code every time you modify a file, but the action of creating a new file doesn't trigger the auto-reloading logic. You can stop it by typing Ctrl-C (Ctrl-Break on Windows); then use the up arrow on your keyboard to find the command again, and hit enter.

[edit] Explore the free admin functionality

Now that we've registered Poll, Django knows that it should be displayed on the admin index page.

Click "Polls." Now you're at the "change list" page for polls. This page displays all the polls in the database and lets you choose one to change it. There's the "What's up?" poll we created in the first tutorial.

Things to note here:

  • The form is automatically generated from the Poll model.
  • The different model field types (DateTimeField, CharField) correspond to the appropriate HTML input widget. Each type of field knows how to display itself in the Django admin.
  • Each DateTimeField gets free JavaScript shortcuts. Dates get a "Today" shortcut and calendar popup, and times get a "Now" shortcut and a convenient popup that lists commonly entered times.

The bottom part of the page gives you a couple of options:

  • Save -- Saves changes and returns to the change-list page for this type of object.
  • Save and continue editing -- Saves changes and reloads the admin page for this object.
  • Save and add another -- Saves changes and loads a new, blank form for this type of object.
  • Delete -- Displays a delete confirmation page.

Change the "Date published" by clicking the "Today" and "Now" shortcuts. Then click "Save and continue editing." Then click "History" in the upper right. You'll see a page listing all changes made to this object via the Django admin, with the timestamp and username of the person who made the change: History page for poll object

[edit] Adding related objects

OK, we have our Poll admin page. But a Poll has multiple Choices, and the admin page doesn't display choices.

Yet.

There are two ways to solve this problem. The first is to register Choice with the admin just as we did with Poll. That's easy:

from polls.models import Choice

admin.site.register(Choice)

Now "Choices" is an available option in the Django admin. Check out the Add Choice form.

In that form, the "Poll" field is a select box containing every poll in the database. Django knows that a ForeignKey should be represented in the admin as a <select> box. In our case, only one poll exists at this point.

Also note the "Add Another" link next to "Poll." Every object with a ForeignKey relationship to another gets this for free. When you click "Add Another," you'll get a popup window with the "Add poll" form. If you add a poll in that window and click "Save," Django will save the poll to the database and dynamically add it as the selected choice on the "Add choice" form you're looking at.

But, really, this is an inefficient way of adding Choice objects to the system. It'd be better if you could add a bunch of Choices directly when you create the Poll object. Let's make that happen.

Remove the register() call for the Choice model. Then, edit the Poll registration code to read:

class ChoiceInline(admin.StackedInline):
    model = Choice
    extra = 3

class PollAdmin(admin.ModelAdmin):
    fieldsets = [
        (None,               {'fields': ['question']}),
        ('Date information', {'fields': ['pub_date'], 'classes': ['collapse']}),
    ]
    inlines = [ChoiceInline]

admin.site.register(Poll, PollAdmin)

This tells Django: "Choice objects are edited on the Poll admin page. By default, provide enough fields for 3 choices."

Load the "Add poll" page to see how that looks, you may need to restart your development server:

It works like this: There are three slots for related Choices -- as specified by extra -- and each time you come back to the "Change" page for an already-created object, you get another three extra slots.

[edit] Customize the admin change list

Now that the Poll admin page is looking good, let's make some tweaks to the admin "change list" page -- the one that displays all the polls in the system.

By default, Django displays the str() of each object. But sometimes it'd be more helpful if we could display individual fields. To do that, use the list_display admin option, which is a tuple of field names to display, as columns, on the change list page for the object:

 class PollAdmin(admin.ModelAdmin):
     # ...
     list_display = ('question', 'pub_date')

Just for good measure, let's also include the was_published_today custom method from Tutorial 1:

class PollAdmin(admin.ModelAdmin):
    # ...
    list_display = ('question', 'pub_date', 'was_published_today')

Now, check out the polls list.

You can click on the column headers to sort by those values -- except in the case of the was_published_today header, because sorting by the output of an arbitrary method is not supported. Also note that the column header for was_published_today is, by default, the name of the method (with underscores replaced with spaces).

This is shaping up well. Let's add some search capability. Add this to class PollAdmin:

class PollAdmin(admin.ModelAdmin):
# ...
    search_fields = ['question']

That adds a search box at the top of the change list. When somebody enters search terms, Django will search the question field. You can use as many fields as you'd like -- although because it uses a LIKE query behind the scenes, keep it reasonable, to keep your database happy.

Finally, because Poll objects have dates, it'd be convenient to be able to drill down by date. Add this line:

class PollAdmin(admin.ModelAdmin):
# ...
    date_hierarchy = 'pub_date'

That adds hierarchical navigation, by date, to the top of the change list page. At top level, it displays all available years. Then it drills down to months and, ultimately, days.

That's the basics of the Django admin interface!

Create a poll! Create some choices. Find your views, and show them to the world.

[edit] Part 4.5: Deploy again, again!

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