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.

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.

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 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:

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

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

>>> Poll.objects.all []

Wait a minute.  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 []

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 [] >>> 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.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)  >>> p.choice_set.create(choice='The sky', votes=0)  >>> c = p.choice_set.create(choice='Just hacking again', votes=0) 

Every Choice can find the Poll that it belongs to:

>>> c.poll 

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 [, , ] >>> 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

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//"></a></li> {% endfor %} </ul> {% 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:

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:

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

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

FIXME: CSS

Part 2.5: 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.



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 element:

{% if error_message %} {% endif %}

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

{% endfor %} <input type="submit" value="Vote" />

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//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, 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:

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

<a href="/polls//">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!

Part 3.5: Deploy again!
Well, your app works! Let's push it to the web so you can send a link to all your friends.

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.

Django comes with a built-in admin interface that lets you add ...

This tutorial begins where Tutorial 1 left off. We’re continuing the Web-poll application and will focus on Django’s automatically-generated admin site.

Philosophy

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. 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:

* 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 mysite/urls.py file and uncomment the lines that reference the admin – there are three lines in total to uncomment. This file is a URLconf; we’ll dig into URLconfs in the next tutorial. For now, all you need to know is that it maps URL roots to applications. In the end, you should have a urls.py file that looks like this:

Changed in Django 1.1: The method for adding admin urls has changed in Django 1.1.

from django.conf.urls.defaults import *

# Uncomment the next two lines to enable the admin: from django.contrib import admin admin.autodiscover

urlpatterns = patterns('',       # Example:        # (r'^mysite/', include('mysite.foo.urls')),

# Uncomment the admin/doc line below and add 'django.contrib.admindocs' # to INSTALLED_APPS to enable admin documentation: # (r'^admin/doc/', include('django.contrib.admindocs.urls')),

# Uncomment the next line to enable the admin: (r'^admin/', include(admin.site.urls)), )

(The bold lines are the ones that needed to be uncommented.)

Start the development server¶

Let’s start the development server and explore the admin site.

Recall from Tutorial 1 that you start the development server like so:

python manage.py runserver

Now, open a Web browser and go to "/admin/" on your local domain -- e.g., http://127.0.0.1:8000/admin/. You should see the admin's login screen: Django admin login screen Enter the admin site¶

Now, try logging in. (You created a superuser account in the first part of this tutorial, remember? If you didn't create one or forgot the password you can create another one.) You should see the Django admin index page: 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. 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. Explore the free admin functionality¶

Now that we've registered Poll, Django knows that it should be displayed on the admin index page: Django admin index page, now with polls displayed

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: Polls change list page

Click the "What's up?" poll to edit it: Editing form for poll object

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 Customize the admin form¶

Take a few minutes to marvel at all the code you didn't have to write. By registering the Poll model with admin.site.register(Poll), Django was able to construct a default form representation. Often, you'll want to customize how the admin form looks and works. You'll do this by telling Django the options you want when you register the object.

Let's see how this works by re-ordering the fields on the edit form. Replace the admin.site.register(Poll) line with:

class PollAdmin(admin.ModelAdmin): fields = ['pub_date', 'question']

admin.site.register(Poll, PollAdmin)

You'll follow this pattern -- create a model admin object, then pass it as the second argument to admin.site.register -- any time you need to change the admin options for an object.

This particular change above makes the "Publication date" come before the "Question" field: Fields have been reordered

This isn't impressive with only two fields, but for admin forms with dozens of fields, choosing an intuitive order is an important usability detail.

And speaking of forms with dozens of fields, you might want to split the form up into fieldsets:

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

admin.site.register(Poll, PollAdmin)

The first element of each tuple in fieldsets is the title of the fieldset. Here's what our form looks like now: Form has fieldsets now

You can assign arbitrary HTML classes to each fieldset. Django provides a "collapse" class that displays a particular fieldset initially collapsed. This is useful when you have a long form that contains a number of fields that aren't commonly used:

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

Fieldset is initially collapsed 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. The "Add choice" form looks like this: Choice admin page

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 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: Add poll page now has choices on it

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.

One small problem, though. It takes a lot of screen space to display all the fields for entering related Choice objects. For that reason, Django offers a tabular way of displaying inline related objects; you just need to change the ChoiceInline declaration to read:

class ChoiceInline(admin.TabularInline): #...

With that TabularInline (instead of StackedInline), the related objects are displayed in a more compact, table-based format: Add poll page now has more compact choices Customize the admin change list¶

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

Here's what it looks like at this point: Polls change list page

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 the poll change list page looks like this: Polls change list page, updated

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). But you can change that by giving that method (in models.py) a short_description attribute:

def was_published_today(self): return self.pub_date.date == datetime.date.today was_published_today.short_description = 'Published today?'

Edit your admin.py file again and add an improvement to the Poll change list page: Filters. Add the following line to PollAdmin:

list_filter = ['pub_date']

That adds a "Filter" sidebar that lets people filter the change list by the pub_date field: Polls change list page, updated

The type of filter displayed depends on the type of field you're filtering on. Because pub_date is a DateTimeField, Django knows to give the default filter options for DateTimeFields: "Any date," "Today," "Past 7 days," "This month," "This year."

This is shaping up well. Let's add some search capability:

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:

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.

Now's also a good time to note that change lists give you free pagination. The default is to display 50 items per page. Change-list pagination, search boxes, filters, date-hierarchies and column-header-ordering all work together like you think they should. Customize the admin look and feel¶

Clearly, having "Django administration" at the top of each admin page is ridiculous. It's just placeholder text.

That's easy to change, though, using Django's template system. The Django admin is powered by Django itself, and its interfaces use Django's own template system.

Open your settings file (mysite/settings.py, remember) and look at the TEMPLATE_DIRS setting. TEMPLATE_DIRS is a tuple of filesystem directories to check when loading Django templates. It's a search path.

By default, TEMPLATE_DIRS is empty. So, let's add a line to it, to tell Django where our templates live:

TEMPLATE_DIRS = (   "/home/my_username/mytemplates", # Change this to your own directory. )

Now copy the template admin/base_site.html from within the default Django admin template directory in the source code of Django itself (django/contrib/admin/templates) into an admin subdirectory of whichever directory you're using in TEMPLATE_DIRS. For example, if your TEMPLATE_DIRS includes "/home/my_username/mytemplates", as above, then copy django/contrib/admin/templates/admin/base_site.html to /home/my_username/mytemplates/admin/base_site.html. Don't forget that admin subdirectory.

Then, just edit the file and replace the generic Django text with your own site's name as you see fit.

This template file contains lots of text like {% block branding %} and. The {% and {{ tags are part of Django's template language. When Django renders admin/base_site.html, this template language will be evaluated to produce the final HTML page. Don't worry if you can't make any sense of the template right now -- we'll delve into Django's templating language in Tutorial 3.

Note that any of Django's default admin templates can be overridden. To override a template, just do the same thing you did with base_site.html -- copy it from the default directory into your custom directory, and make changes.

Astute readers will ask: But if TEMPLATE_DIRS was empty by default, how was Django finding the default admin templates? The answer is that, by default, Django automatically looks for a templates/ subdirectory within each app package, for use as a fallback. See the template loader documentation for full information. Customize the admin index page¶

On a similar note, you might want to customize the look and feel of the Django admin index page.

By default, it displays all the apps in INSTALLED_APPS that have been registered with the admin application, in alphabetical order. You may want to make significant changes to the layout. After all, the index is probably the most important page of the admin, and it should be easy to use.

The template to customize is admin/index.html. (Do the same as with admin/base_site.html in the previous section -- copy it from the default directory to your custom template directory.) Edit the file, and you'll see it uses a template variable called app_list. That variable contains every installed Django app. Instead of using that, you can hard-code links to object-specific admin pages in whatever way you think is best. Again, don't worry if you can't understand the template language -- we'll cover that in more detail in Tutorial 3.