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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.
Line 14 ⟶ 8:
 
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.
 
== Writing your first Django app, part 1 ==
Line 19 ⟶ 15:
Let’s learn by example.
 
Throughout thisThis tutorial, we’ll walkwalks you through the creation of a basic poll application.
 
It’ll consist of two parts:
Line 44 ⟶ 40:
=== Look at the files ===
 
Let’s look at files are in the project:
 
workshop_mysite/
public/
README.mediawiki
__init__.py
manage.py
Line 54 ⟶ 52:
These files are:
 
* README.mediawiki: Many projects come with ''README'' files that, well, you should read. This one does, too.
* __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.)
* 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.
Line 61:
=== The development server ===
 
Let's verify this worked. If you haven't already, and runRun the command python manage.py runserver. You'll see the following output on the command line:
 
<pre>
python manage.py runserver
</pre>
 
You'll see the following output on the command line:
 
<pre>
Validating models...
0 errors found.
 
Django version 1.02, using settings 'mysite.settings'
Development server is running at http://127.0.0.1:8000/
Quit the server with CONTROL-C.
</pre>
 
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.
 
=== Fixing security settings ===
Line 84 ⟶ 94:
=== Database setup ===
 
Keep looking at settings.py: itThe hasDATABASES 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.
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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'' 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.
 
Line 146 ⟶ 155:
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.
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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
Line 160 ⟶ 169:
class Poll(models.Model):
question = models.CharField(max_length=200)
pub_date = models.DateTimeField('date published')
class Choice(models.Model):
Line 166 ⟶ 175:
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.
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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.
Line 201 ⟶ 210:
'polls',
)
 
Save the settings.py file.
 
Now Django knows to include the polls app.
Line 215 ⟶ 226:
 
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 ===
Line 276 ⟶ 281:
[<Poll: Poll object>]
 
Wait a minute. <Poll: Poll object> is, an 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:
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):
Line 302 ⟶ 308:
 
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
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If you want to search your database, you can do it using the '''filter''' method on the ''objects'' attribute of Poll. For example:
 
>>> ppolls = Poll.objects.filter(question="What's up?")
>>> ppolls
[<Poll: What's up?>]
>>> ppolls[0].id
1
 
Line 336 ⟶ 343:
Right now, we have a Poll in the database, but it has no Choices. See:
 
>>> p = Poll.objects.get(pkid=1)
>>> p.choice_set.all()
[]
Line 347 ⟶ 354:
<Choice: The sky>
>>> c = p.choice_set.create(choice='Just hacking again', votes=0)
>>> c
<Choice: Just hacking again>
 
Line 363 ⟶ 371:
=== 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.)
* FIXME
 
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.
 
=== 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!
 
=== Enough databases for now ===
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=== Adding URLs to urls.py ===
 
When we ran django-admin.py startproject workshop_mysite to create the project, Django created a default URLconf in workshop_mysite/urls.py. ItTake alsoa automaticallylook set your ROOT_URLCONF setting (inat '''settings.py)''' to point atfor thatthis fileline:
 
ROOT_URLCONF = 'workshop_mysite.urls'
 
That means that the default URLconf is workshop_mysite/urls.py.
Time for an example. Edit mysite/urls.py so it looks like this:
 
Time for an example. Edit the file workshop_mysite/urls.py so it looks like this:
 
<pre>
from django.conf.urls.defaults import *
 
Line 429 ⟶ 485:
(r'^polls/(\d+)/vote/$', 'polls.views.vote'),
)
</pre>
This is worth a review. When somebody requests a page from your Web site -- say, "/polls/23/", Django will load the ''urls.py'' Python module, because it's pointed to by the ROOT_URLCONF setting. It finds the variable named urlpatterns and traverses the regular expressions in order. When it finds a regular expression that matches -- r'^polls/(\d+)/$' -- it loads the function detail() from polls/views.py. Finally, it calls that detail() function like so:
 
detail(request=<HttpRequest object>, '23')
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/(?P<poll_id>\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).
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python manage.py runserver
 
Now go to "http://localhost:8000/polls/" on your domain in your Web browser. You should get a pleasantly-colored error page with the following message:
 
ViewDoesNotExist at /polls/
Line 460 ⟶ 516:
 
from django.http import HttpResponse
 
def index(request):
return HttpResponse("Hello, world. You're at the poll index.")
 
This is the simplest view possible. GoSave the views.py file, then go to "/polls/" in your browser, and you should see your text.
 
Now letslet'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)
 
TakeSave 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.
 
=== Write views that actually do something ===
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All Django wants is that HttpResponse. Or an exception.
 
BecauseMost it'sof convenient,the let'sDjango 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]
Line 497 ⟶ 553:
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.templateshortcuts import Context, loaderrender_to_response
from polls.models import Poll
from django.http import HttpResponse
 
def index(request):
latest_poll_list = Poll.objects.all().order_by('-pub_date')[:5]
context = {'latest_poll_list': latest_poll_list}
t = loader.get_template('polls/index.html')
return render_to_response('polls/index.html', context)
c = Context({
'latest_poll_list': latest_poll_list,
})
return HttpResponse(t.render(c))
 
To recap what this does:
That code loads the template called "polls/index.html" and passes it a context. The context is a dictionary mapping template variable names to Python objects.
 
* 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:
Line 518 ⟶ 578:
polls/index.html
 
Ah. There's no template yet. Let's make one.
Ah. There's no template yet. First, create a directory, somewhere on your filesystem, whose contents Django can access. (Django runs as whatever user your server runs.) Don't put them under your document root, though. You probably shouldn't make them public, just for security's sake. Then edit TEMPLATE_DIRS in your settings.py to tell Django where it can find templates -- just as you did in the "Customize the admin look and feel" section of Tutorial 2.
 
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:
When you've done that, create a directory polls in your template directory. Within that, create a file called index.html. Note that our loader.get_template('polls/index.html') code from above maps to "[template_directory]/polls/index.html" on the filesystem.
 
mkdir -p polls/templates/polls
 
Within that, create a file called index.html.
 
Put the following code in that template:
 
<pre>
{% if latest_poll_list %}
{% if latest_poll_list %}
<ul>
<ul>
{% for poll in latest_poll_list %}
{% for poll in latest_poll_list %}
<li><a href="/polls/{{ poll.id }}/">{{ poll.question }}</a></li>
<li><a href="/polls/{{ poll.id }}/">{{ poll.question }}</a></li>
{% endfor %}
</ul> {% endfor %}
</ul>
{% else %}
{% else %}
<p>No polls are available.</p>
<p>No polls are available.</p>
{% endif %}
{% endif %}
</pre>
 
Load the page in"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.
A shortcut: render_to_response()¶
 
=== Raising 404 ===
It's a very common idiom to load a template, fill a context and return an HttpResponse object with the result of the rendered template. Django provides a shortcut. Here's the full index() view, rewritten:
 
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.shortcuts import render_to_response
from polls.models import Poll
 
from django.http import Http404
def index(request):
# ...
latest_poll_list = Poll.objects.all().order_by('-pub_date')[:5]
def detail(request, poll_id):
return render_to_response('polls/index.html', {'latest_poll_list': latest_poll_list})
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.
Note that once we've done this in all these views, we no longer need to import loader, Context and HttpResponse.
 
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:
The render_to_response() function takes a template name as its first argument and a dictionary as its optional second argument. It returns an HttpResponse object of the given template rendered with the given context.
Raising 404¶
 
<pre>
Now, let's tackle the poll detail view -- the page that displays the question for a given poll. Here's the view:
{{ poll }}
</pre>
 
to get you started for now.
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})
 
Does your detail view work? Try it: http://127.0.0.1:8000/polls/1/
The new concept here: The view raises the Http404 exception if a poll with the requested ID doesn't exist.
 
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/
We'll discuss what you could put in that polls/detail.html template a bit later, but if you'd like to quickly get the above example working, just:
 
=== Adding more detail ===
{{ poll }}
 
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:
 
<pre>
<h1>{{ poll.question }}</h1>
<ul>
{% for choice in poll.choice_set.all %}
<li>{{ choice.choice }}</li>
{% endfor %}
</ul>
</pre>
 
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.
 
=== Adding some style ===
 
The web page looks okay, but it is somewhat drab.
 
FIXME: CSS
 
== Part 3: Let people vote ==
 
=== 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:
 
<pre>
<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>
</pre>
 
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.
will get you started for now.
* We set the form's action to <pre>/polls/{{ poll.id }}/vote/</pre>, 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.)
A shortcut: get_object_or_404()¶
* 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:
It's a very common idiom to use get() and raise Http404 if the object doesn't exist. Django provides a shortcut. Here's the detail() view, rewritten:
 
<pre>
from django.shortcuts import render_to_response, get_object_or_404
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))
</pre>
 
The details of how this works are explained in the [http://docs.djangoproject.com/en/dev/ref/templates/api/#subclassing-context-requestcontext documentation for RequestContext].
The get_object_or_404() function takes a Django model as its first argument and an arbitrary number of keyword arguments, which it passes to the module's get() function. It raises Http404 if the object doesn't exist.
 
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:
Philosophy
 
(r'^(?P<poll_id>\d+)/vote/$', 'vote'),
Why do we use a helper function get_object_or_404() instead of automatically catching the ObjectDoesNotExist exceptions at a higher level, or having the model API raise Http404 instead of ObjectDoesNotExist?
 
We also created a dummy implementation of the vote() function. Let's create a real version. Add the following to polls/views.py:
Because that would couple the model layer to the view layer. One of the foremost design goals of Django is to maintain loose coupling.
 
from django.shortcuts import get_object_or_404, render_to_response
There's also a get_list_or_404() function, which works just as get_object_or_404() -- except using filter() instead of get(). It raises Http404 if the list is empty.
from django.http import HttpResponseRedirect, HttpResponse
Write a 404 (page not found) view¶
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:
When you raise Http404 from within a view, Django will load a special view devoted to handling 404 errors. It finds it by looking for the variable handler404, which is a string in Python dotted syntax -- the same format the normal URLconf callbacks use. A 404 view itself has nothing special: It's just a normal view.
 
* 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.
You normally won't have to bother with writing 404 views. By default, URLconfs have the following line up top:
* 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.
from django.conf.urls.defaults import *
 
That takes care of setting handler404 in the current module. As you can see in django/conf/urls/defaults.py, handler404 is set to django.views.defaults.page_not_found() by default.
 
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
Four more things to note about 404 views:
 
'/polls/3/results/'
* If DEBUG is set to True (in your settings module) then your 404 view will never be used (and thus the 404.html template will never be rendered) because the traceback will be displayed instead.
* The 404 view is also called if Django doesn't find a match after checking every regular expression in the URLconf.
* If you don't define your own 404 view -- and simply use the default, which is recommended -- you still have one obligation: To create a 404.html template in the root of your template directory. The default 404 view will use that template for all 404 errors.
* If DEBUG is set to False (in your settings module) and if you didn't create a 404.html file, an Http500 is raised instead. So remember to create a 404.html.
 
... 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).
Write a 500 (server error) view¶
 
After somebody votes in a poll, the vote() view redirects to the results page for the poll. Let's write that view:
Similarly, URLconfs may define a handler500, which points to a view to call in case of server errors. Server errors happen when you have runtime errors in view code.
Use the template system¶
 
def results(request, poll_id):
Back to the detail() view for our poll application. Given the context variable poll, here's what the "polls/detail.html" template might look like:
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:
 
<pre>
<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>
The template system uses dot-lookup syntax to access variable attributes. In the example of {{ poll.question }}, first Django does a dictionary lookup on the object poll. Failing that, it tries an attribute lookup -- which works, in this case. If attribute lookup had failed, it would've tried a list-index lookup.
</pre>
 
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.
Method-calling happens in the {% for %} loop: poll.choice_set.all is interpreted as the Python code poll.choice_set.all(), which returns an iterable of Choice objects and is suitable for use in the {% for %} tag.
 
Does it work?! If so, show your neighbor!
See the template guide for more about templates.
Simplifying the URLconfs¶
 
== Part 3.5: Deploy your web app! ==
Take some time to play around with the views and template system. As you edit the URLconf, you may notice there's a fair bit of redundancy in it:
 
You've done a lot of work. It's time to share it with the world.
urlpatterns = patterns('',
(r'^polls/$', 'polls.views.index'),
(r'^polls/(?P<poll_id>\d+)/$', 'polls.views.detail'),
(r'^polls/(?P<poll_id>\d+)/results/$', 'polls.views.results'),
(r'^polls/(?P<poll_id>\d+)/vote/$', 'polls.views.vote'),
)
 
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.
Namely, polls.views is in every callback.
 
You've already pushed some work to Github. To get our changes over to alwaysdata. you'll:
Because this is a common case, the URLconf framework provides a shortcut for common prefixes. You can factor out the common prefixes and add them as the first argument to patterns(), like so:
 
# Add and commit files on your own computer.
urlpatterns = patterns('polls.views',
# Push your changes to Github.
(r'^polls/$', 'index'),
# Connect to your alwaysdata.com account via SSH/PuTTY
(r'^polls/(?P<poll_id>\d+)/$', 'detail'),
# Run "git pull" to get the latest version to your Alwaysdata account.
(r'^polls/(?P<poll_id>\d+)/results/$', 'results'),
(r'^polls/(?P<poll_id>\d+)/vote/$', 'vote'),
)
 
So we'll do those steps in order.
This is functionally identical to the previous formatting. It's just a bit tidier.
 
To do the ''add and commit'', open up your Terminal or GitBash:
Since you generally don't want the prefix for one app to be applied to every callback in your URLconf, you can concatenate multiple patterns(). Your full mysite/urls.py might now look like this:
 
git add .
from django.conf.urls.defaults import *
git commit -m "More changes"
 
To push:
from django.contrib import admin
admin.autodiscover()
 
git push
urlpatterns = patterns('polls.views',
(r'^polls/$', 'index'),
(r'^polls/(?P<poll_id>\d+)/$', 'detail'),
(r'^polls/(?P<poll_id>\d+)/results/$', 'results'),
(r'^polls/(?P<poll_id>\d+)/vote/$', 'vote'),
)
 
Now, open up SSH or PuTTY and connect to your alwaysdata.com account.
urlpatterns += patterns('',
(r'^admin/', include(admin.site.urls)),
)
 
Finally, in '''that''' terminal:
Decoupling the URLconfs¶
 
cd workshop_mysite
While we're at it, we should take the time to decouple our poll-app URLs from our Django project configuration. Django apps are meant to be pluggable -- that is, each particular app should be transferable to another Django installation with minimal fuss.
git pull
 
Okay, not quite finally. You might need to go to https://admin.alwaysdata.com/advanced/processes/ and click ''Restart my applications''.
Our poll app is pretty decoupled at this point, thanks to the strict directory structure that python manage.py startapp created, but one part of it is coupled to the Django settings: The URLconf.
 
Go to your alwaysdata site's /polls/ page. For me, I'd go to:
We've been editing the URLs in mysite/urls.py, but the URL design of an app is specific to the app, not to the Django installation -- so let's move the URLs within the app directory.
 
* http://paulproteus.alwaysdata.com/polls/
Copy the file mysite/urls.py to polls/urls.py. Then, change mysite/urls.py to remove the poll-specific URLs and insert an include(), leaving you with:
 
You should see your poll!
# This also imports the include function
from django.conf.urls.defaults import *
 
== 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.
 
=== 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.
 
=== 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.
 
=== 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.
=== 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.
 
=== 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:
 
<pre>
from polls.models import Poll
from django.contrib import admin
admin.autodiscover()
 
admin.site.register(Poll)
urlpatterns = patterns('',
</pre>
(r'^polls/', include('polls.urls')),
(r'^admin/', include(admin.site.urls)),
)
 
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.
include() simply references another URLconf. Note that the regular expression doesn't have a $ (end-of-string match character) but has the trailing slash. Whenever Django encounters include(), it chops off whatever part of the URL matched up to that point and sends the remaining string to the included URLconf for further processing.
 
=== Explore the free admin functionality ===
Here's what happens if a user goes to "/polls/34/" in this system:
 
Now that we've registered Poll, Django knows that it should be displayed on the admin index page.
* Django will find the match at '^polls/'
* Then, Django will strip off the matching text ("polls/") and send the remaining text -- "34/" -- to the 'polls.urls' URLconf for further processing.
 
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.
Now that we've decoupled that, we need to decouple the polls.urls URLconf by removing the leading "polls/" from each line, and removing the lines registering the admin site. Your polls.urls file should now look like this:
 
Things to note here:
from django.conf.urls.defaults import *
 
* The form is automatically generated from the Poll model.
urlpatterns = patterns('polls.views',
* 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.
(r'^$', 'index'),
* Each DateTimeField gets free JavaScript shortcuts. Dates get a "Today" shortcut and calendar popup, and times get a "Now" shortcut and a convenient popup that lists commonly entered times.
(r'^(?P<poll_id>\d+)/$', 'detail'),
(r'^(?P<poll_id>\d+)/results/$', 'results'),
(r'^(?P<poll_id>\d+)/vote/$', 'vote'),
)
 
The bottom part of the page gives you a couple of options:
The idea behind include() and URLconf decoupling is to make it easy to plug-and-play URLs. Now that polls are in their own URLconf, they can be placed under "/polls/", or under "/fun_polls/", or under "/content/polls/", or any other path root, and the app will still work.
 
* Save -- Saves changes and returns to the change-list page for this type of object.
All the poll app cares about is its relative path, not its absolute path.
* 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:
When you're comfortable with writing views, read part 4 of this tutorial to learn about simple form processing and generic views.
History page for poll object
 
=== Adding related objects ===
== Part 2.5: Deploy your web app! ==
 
OK, we have our Poll admin page. But a Poll has multiple Choices, and the admin page doesn't display choices.
== Part 3: Let people vote ==
 
Yet.
== Part 3.5: Deploy again! ==
 
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:
== Part 4: Editing your polls in the Django admin interface ==
 
<pre>
from polls.models import Choice
 
admin.site.register(Choice)
</pre>
 
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:
 
<pre>
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)
</pre>
 
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.
 
=== 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:
 
<pre>
class PollAdmin(admin.ModelAdmin):
# ...
list_display = ('question', 'pub_date')
</pre>
 
Just for good measure, let's also include the was_published_today custom method from Tutorial 1:
 
<pre>
class PollAdmin(admin.ModelAdmin):
# ...
list_display = ('question', 'pub_date', 'was_published_today')
</pre>
 
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.
 
== Part 4.5: Deploy again, again! ==
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