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<div class="section" id="s-writing-your-first-django-app-part-3">
<span id="writing-your-first-django-app-part-3"></span><h1>Writing your first Django app, part 3<a class="headerlink" href="#writing-your-first-django-app-part-3" title="Permalink to this headline">¶</a></h1>
<p>This tutorial begins where <a class="reference internal" href="tutorial02.html"><em>Tutorial 2</em></a> left off. We’re
continuing the Web-poll application and will focus on creating the public
interface – “views.”</p>
<div class="section" id="s-philosophy">
<span id="philosophy"></span><h2>Philosophy<a class="headerlink" href="#philosophy" title="Permalink to this headline">¶</a></h2>
<p>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:</p>
<ul class="simple">
<li>Blog homepage – displays the latest few entries.</li>
<li>Entry “detail” page – permalink page for a single entry.</li>
<li>Year-based archive page – displays all months with entries in the
given year.</li>
<li>Month-based archive page – displays all days with entries in the
given month.</li>
<li>Day-based archive page – displays all entries in the given day.</li>
<li>Comment action – handles posting comments to a given entry.</li>
</ul>
<p>In our poll application, we’ll have the following four views:</p>
<ul class="simple">
<li>Poll “index” page – displays the latest few polls.</li>
<li>Poll “detail” page – displays a poll question, with no results but
with a form to vote.</li>
<li>Poll “results” page – displays results for a particular poll.</li>
<li>Vote action – handles voting for a particular choice in a particular
poll.</li>
</ul>
<p>In Django, each view is represented by a simple Python function.</p>
</div>
<div class="section" id="s-design-your-urls">
<span id="design-your-urls"></span><h2>Design your URLs<a class="headerlink" href="#design-your-urls" title="Permalink to this headline">¶</a></h2>
<p>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.</p>
<p>When a user requests a Django-powered page, the system looks at the
<a class="reference internal" href="../ref/settings.html#std:setting-ROOT_URLCONF"><tt class="xref std std-setting docutils literal"><span class="pre">ROOT_URLCONF</span></tt></a> setting, which contains a string in Python dotted
syntax. Django loads that module and looks for a module-level variable called
<tt class="docutils literal"><span class="pre">urlpatterns</span></tt>, which is a sequence of tuples in the following format:</p>
<div class="highlight-python"><pre>(regular expression, Python callback function [, optional dictionary])</pre>
</div>
<p>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.</p>
<p>When it finds a match, Django calls the Python callback function, with an
<a class="reference internal" href="../ref/request-response.html#django.http.HttpRequest" title="django.http.HttpRequest"><tt class="xref py py-class docutils literal"><span class="pre">HttpRequest</span></tt></a> object as the first argument, any “captured”
values from the regular expression as keyword arguments, and, optionally,
arbitrary keyword arguments from the dictionary (an optional third item in the
tuple).</p>
<p>For more on <a class="reference internal" href="../ref/request-response.html#django.http.HttpRequest" title="django.http.HttpRequest"><tt class="xref py py-class docutils literal"><span class="pre">HttpRequest</span></tt></a> objects, see the
<a class="reference internal" href="../ref/request-response.html"><em>Request and response objects</em></a>. For more details on URLconfs, see the
<a class="reference internal" href="../topics/http/urls.html"><em>URL dispatcher</em></a>.</p>
<p>When you ran <tt class="docutils literal"><span class="pre">django-admin.py</span> <span class="pre">startproject</span> <span class="pre">mysite</span></tt> at the beginning of
Tutorial 1, it created a default URLconf in <tt class="docutils literal"><span class="pre">mysite/urls.py</span></tt>. It also
automatically set your <a class="reference internal" href="../ref/settings.html#std:setting-ROOT_URLCONF"><tt class="xref std std-setting docutils literal"><span class="pre">ROOT_URLCONF</span></tt></a> setting (in <tt class="docutils literal"><span class="pre">settings.py</span></tt>) to
point at that file:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">ROOT_URLCONF</span> <span class="o">=</span> <span class="s">'mysite.urls'</span>
</pre></div>
</div>
<p>Time for an example. Edit <tt class="docutils literal"><span class="pre">mysite/urls.py</span></tt> so it looks like this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">django.conf.urls</span> <span class="kn">import</span> <span class="n">patterns</span><span class="p">,</span> <span class="n">include</span><span class="p">,</span> <span class="n">url</span>
<span class="kn">from</span> <span class="nn">django.contrib</span> <span class="kn">import</span> <span class="n">admin</span>
<span class="n">admin</span><span class="o">.</span><span class="n">autodiscover</span><span class="p">()</span>
<span class="n">urlpatterns</span> <span class="o">=</span> <span class="n">patterns</span><span class="p">(</span><span class="s">''</span><span class="p">,</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/$'</span><span class="p">,</span> <span class="s">'polls.views.index'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/(?P<poll_id>\d+)/$'</span><span class="p">,</span> <span class="s">'polls.views.detail'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/(?P<poll_id>\d+)/results/$'</span><span class="p">,</span> <span class="s">'polls.views.results'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/(?P<poll_id>\d+)/vote/$'</span><span class="p">,</span> <span class="s">'polls.views.vote'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^admin/'</span><span class="p">,</span> <span class="n">include</span><span class="p">(</span><span class="n">admin</span><span class="o">.</span><span class="n">site</span><span class="o">.</span><span class="n">urls</span><span class="p">)),</span>
<span class="p">)</span>
</pre></div>
</div>
<p>This is worth a review. When somebody requests a page from your Web site – say,
“/polls/23/”, Django will load this Python module, because it’s pointed to by
the <a class="reference internal" href="../ref/settings.html#std:setting-ROOT_URLCONF"><tt class="xref std std-setting docutils literal"><span class="pre">ROOT_URLCONF</span></tt></a> setting. It finds the variable named <tt class="docutils literal"><span class="pre">urlpatterns</span></tt>
and traverses the regular expressions in order. When it finds a regular
expression that matches – <tt class="docutils literal"><span class="pre">r'^polls/(?P<poll_id>\d+)/$'</span></tt> – it loads the
function <tt class="docutils literal"><span class="pre">detail()</span></tt> from <tt class="docutils literal"><span class="pre">polls/views.py</span></tt>. Finally, it calls that
<tt class="docutils literal"><span class="pre">detail()</span></tt> function like so:</p>
<div class="highlight-python"><pre>detail(request=<HttpRequest object>, poll_id='23')</pre>
</div>
<p>The <tt class="docutils literal"><span class="pre">poll_id='23'</span></tt> part comes from <tt class="docutils literal"><span class="pre">(?P<poll_id>\d+)</span></tt>. Using parentheses
around a pattern “captures” the text matched by that pattern and sends it as an
argument to the view function; the <tt class="docutils literal"><span class="pre">?P<poll_id></span></tt> defines the name that will be
used to identify the matched pattern; and <tt class="docutils literal"><span class="pre">\d+</span></tt> is a regular expression to
match a sequence of digits (i.e., a number).</p>
<p>Because the URL patterns are regular expressions, there really is no limit on
what you can do with them. And there’s no need to add URL cruft such as <tt class="docutils literal"><span class="pre">.php</span></tt>
– unless you have a sick sense of humor, in which case you can do something
like this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="p">(</span><span class="s">r'^polls/latest\.php$'</span><span class="p">,</span> <span class="s">'polls.views.index'</span><span class="p">),</span>
</pre></div>
</div>
<p>But, don’t do that. It’s silly.</p>
<p>Note that these regular expressions do not search GET and POST parameters, or
the domain name. For example, in a request to <tt class="docutils literal"><span class="pre">http://www.example.com/myapp/</span></tt>,
the URLconf will look for <tt class="docutils literal"><span class="pre">myapp/</span></tt>. In a request to
<tt class="docutils literal"><span class="pre">http://www.example.com/myapp/?page=3</span></tt>, the URLconf will look for <tt class="docutils literal"><span class="pre">myapp/</span></tt>.</p>
<p>If you need help with regular expressions, see <a class="reference external" href="http://en.wikipedia.org/wiki/Regular_expression">Wikipedia’s entry</a> and the
documentation of the <tt class="xref py py-mod docutils literal"><span class="pre">re</span></tt> module. Also, the O’Reilly book “Mastering
Regular Expressions” by Jeffrey Friedl is fantastic.</p>
<p>Finally, a performance note: these regular expressions are compiled the first
time the URLconf module is loaded. They’re super fast.</p>
</div>
<div class="section" id="s-write-your-first-view">
<span id="write-your-first-view"></span><h2>Write your first view<a class="headerlink" href="#write-your-first-view" title="Permalink to this headline">¶</a></h2>
<p>Well, we haven’t created any views yet – we just have the URLconf. But let’s
make sure Django is following the URLconf properly.</p>
<p>Fire up the Django development Web server:</p>
<div class="highlight-bash"><div class="highlight"><pre>python manage.py runserver
</pre></div>
</div>
<p>Now go to “<a class="reference external" href="http://localhost:8000/polls/">http://localhost:8000/polls/</a>” on your domain in your Web browser.
You should get a pleasantly-colored error page with the following message:</p>
<div class="highlight-python"><pre>ViewDoesNotExist at /polls/
Could not import polls.views.index. View does not exist in module polls.views.</pre>
</div>
<p>This error happened because you haven’t written a function <tt class="docutils literal"><span class="pre">index()</span></tt> in the
module <tt class="docutils literal"><span class="pre">polls/views.py</span></tt>.</p>
<p>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).</p>
<p>Time to write the first view. Open the file <tt class="docutils literal"><span class="pre">polls/views.py</span></tt>
and put the following Python code in it:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">django.http</span> <span class="kn">import</span> <span class="n">HttpResponse</span>
<span class="k">def</span> <span class="nf">index</span><span class="p">(</span><span class="n">request</span><span class="p">):</span>
<span class="k">return</span> <span class="n">HttpResponse</span><span class="p">(</span><span class="s">"Hello, world. You're at the poll index."</span><span class="p">)</span>
</pre></div>
</div>
<p>This is the simplest view possible. Go to “/polls/” in your browser, and you
should see your text.</p>
<p>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):</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">detail</span><span class="p">(</span><span class="n">request</span><span class="p">,</span> <span class="n">poll_id</span><span class="p">):</span>
<span class="k">return</span> <span class="n">HttpResponse</span><span class="p">(</span><span class="s">"You're looking at poll </span><span class="si">%s</span><span class="s">."</span> <span class="o">%</span> <span class="n">poll_id</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">results</span><span class="p">(</span><span class="n">request</span><span class="p">,</span> <span class="n">poll_id</span><span class="p">):</span>
<span class="k">return</span> <span class="n">HttpResponse</span><span class="p">(</span><span class="s">"You're looking at the results of poll </span><span class="si">%s</span><span class="s">."</span> <span class="o">%</span> <span class="n">poll_id</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">vote</span><span class="p">(</span><span class="n">request</span><span class="p">,</span> <span class="n">poll_id</span><span class="p">):</span>
<span class="k">return</span> <span class="n">HttpResponse</span><span class="p">(</span><span class="s">"You're voting on poll </span><span class="si">%s</span><span class="s">."</span> <span class="o">%</span> <span class="n">poll_id</span><span class="p">)</span>
</pre></div>
</div>
<p>Take a look in your browser, at “/polls/34/”. It’ll run the <cite>detail()</cite> 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.</p>
</div>
<div class="section" id="s-write-views-that-actually-do-something">
<span id="write-views-that-actually-do-something"></span><h2>Write views that actually do something<a class="headerlink" href="#write-views-that-actually-do-something" title="Permalink to this headline">¶</a></h2>
<p>Each view is responsible for doing one of two things: Returning an
<a class="reference internal" href="../ref/request-response.html#django.http.HttpResponse" title="django.http.HttpResponse"><tt class="xref py py-class docutils literal"><span class="pre">HttpResponse</span></tt></a> object containing the content for the
requested page, or raising an exception such as <a class="reference internal" href="../topics/http/views.html#django.http.Http404" title="django.http.Http404"><tt class="xref py py-exc docutils literal"><span class="pre">Http404</span></tt></a>. The
rest is up to you.</p>
<p>Your view can read records from a database, or not. It can use a template
system such as Django’s – or a third-party Python template system – 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.</p>
<p>All Django wants is that <a class="reference internal" href="../ref/request-response.html#django.http.HttpResponse" title="django.http.HttpResponse"><tt class="xref py py-class docutils literal"><span class="pre">HttpResponse</span></tt></a>. Or an exception.</p>
<p>Because it’s convenient, let’s use Django’s own database API, which we covered
in <a class="reference internal" href="tutorial01.html"><em>Tutorial 1</em></a>. Here’s one stab at the <tt class="docutils literal"><span class="pre">index()</span></tt>
view, which displays the latest 5 poll questions in the system, separated by
commas, according to publication date:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">polls.models</span> <span class="kn">import</span> <span class="n">Poll</span>
<span class="kn">from</span> <span class="nn">django.http</span> <span class="kn">import</span> <span class="n">HttpResponse</span>
<span class="k">def</span> <span class="nf">index</span><span class="p">(</span><span class="n">request</span><span class="p">):</span>
<span class="n">latest_poll_list</span> <span class="o">=</span> <span class="n">Poll</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">all</span><span class="p">()</span><span class="o">.</span><span class="n">order_by</span><span class="p">(</span><span class="s">'-pub_date'</span><span class="p">)[:</span><span class="mi">5</span><span class="p">]</span>
<span class="n">output</span> <span class="o">=</span> <span class="s">', '</span><span class="o">.</span><span class="n">join</span><span class="p">([</span><span class="n">p</span><span class="o">.</span><span class="n">question</span> <span class="k">for</span> <span class="n">p</span> <span class="ow">in</span> <span class="n">latest_poll_list</span><span class="p">])</span>
<span class="k">return</span> <span class="n">HttpResponse</span><span class="p">(</span><span class="n">output</span><span class="p">)</span>
</pre></div>
</div>
<p>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:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">django.template</span> <span class="kn">import</span> <span class="n">Context</span><span class="p">,</span> <span class="n">loader</span>
<span class="kn">from</span> <span class="nn">polls.models</span> <span class="kn">import</span> <span class="n">Poll</span>
<span class="kn">from</span> <span class="nn">django.http</span> <span class="kn">import</span> <span class="n">HttpResponse</span>
<span class="k">def</span> <span class="nf">index</span><span class="p">(</span><span class="n">request</span><span class="p">):</span>
<span class="n">latest_poll_list</span> <span class="o">=</span> <span class="n">Poll</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">all</span><span class="p">()</span><span class="o">.</span><span class="n">order_by</span><span class="p">(</span><span class="s">'-pub_date'</span><span class="p">)[:</span><span class="mi">5</span><span class="p">]</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">loader</span><span class="o">.</span><span class="n">get_template</span><span class="p">(</span><span class="s">'polls/index.html'</span><span class="p">)</span>
<span class="n">c</span> <span class="o">=</span> <span class="n">Context</span><span class="p">({</span>
<span class="s">'latest_poll_list'</span><span class="p">:</span> <span class="n">latest_poll_list</span><span class="p">,</span>
<span class="p">})</span>
<span class="k">return</span> <span class="n">HttpResponse</span><span class="p">(</span><span class="n">t</span><span class="o">.</span><span class="n">render</span><span class="p">(</span><span class="n">c</span><span class="p">))</span>
</pre></div>
</div>
<p>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.</p>
<p>Reload the page. Now you’ll see an error:</p>
<div class="highlight-python"><pre>TemplateDoesNotExist at /polls/
polls/index.html</pre>
</div>
<p>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 <a class="reference internal" href="../ref/settings.html#std:setting-TEMPLATE_DIRS"><tt class="xref std std-setting docutils literal"><span class="pre">TEMPLATE_DIRS</span></tt></a> in your <tt class="docutils literal"><span class="pre">settings.py</span></tt> to tell Django where
it can find templates – just as you did in the “Customize the admin look and
feel” section of Tutorial 2.</p>
<p>When you’ve done that, create a directory <tt class="docutils literal"><span class="pre">polls</span></tt> in your template directory.
Within that, create a file called <tt class="docutils literal"><span class="pre">index.html</span></tt>. Note that our
<tt class="docutils literal"><span class="pre">loader.get_template('polls/index.html')</span></tt> code from above maps to
“[template_directory]/polls/index.html” on the filesystem.</p>
<p>Put the following code in that template:</p>
<div class="highlight-html+django"><div class="highlight"><pre><span class="cp">{%</span> <span class="k">if</span> <span class="nv">latest_poll_list</span> <span class="cp">%}</span>
<span class="nt"><ul></span>
<span class="cp">{%</span> <span class="k">for</span> <span class="nv">poll</span> <span class="k">in</span> <span class="nv">latest_poll_list</span> <span class="cp">%}</span>
<span class="nt"><li><a</span> <span class="na">href=</span><span class="s">"/polls/</span><span class="cp">{{</span> <span class="nv">poll.id</span> <span class="cp">}}</span><span class="s">/"</span><span class="nt">></span><span class="cp">{{</span> <span class="nv">poll.question</span> <span class="cp">}}</span><span class="nt"></a></li></span>
<span class="cp">{%</span> <span class="k">endfor</span> <span class="cp">%}</span>
<span class="nt"></ul></span>
<span class="cp">{%</span> <span class="k">else</span> <span class="cp">%}</span>
<span class="nt"><p></span>No polls are available.<span class="nt"></p></span>
<span class="cp">{%</span> <span class="k">endif</span> <span class="cp">%}</span>
</pre></div>
</div>
<p>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.</p>
<div class="section" id="s-a-shortcut-render-to-response">
<span id="a-shortcut-render-to-response"></span><h3>A shortcut: render_to_response()<a class="headerlink" href="#a-shortcut-render-to-response" title="Permalink to this headline">¶</a></h3>
<p>It’s a very common idiom to load a template, fill a context and return an
<a class="reference internal" href="../ref/request-response.html#django.http.HttpResponse" title="django.http.HttpResponse"><tt class="xref py py-class docutils literal"><span class="pre">HttpResponse</span></tt></a> object with the result of the rendered
template. Django provides a shortcut. Here’s the full <tt class="docutils literal"><span class="pre">index()</span></tt> view,
rewritten:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">django.shortcuts</span> <span class="kn">import</span> <span class="n">render_to_response</span>
<span class="kn">from</span> <span class="nn">polls.models</span> <span class="kn">import</span> <span class="n">Poll</span>
<span class="k">def</span> <span class="nf">index</span><span class="p">(</span><span class="n">request</span><span class="p">):</span>
<span class="n">latest_poll_list</span> <span class="o">=</span> <span class="n">Poll</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">all</span><span class="p">()</span><span class="o">.</span><span class="n">order_by</span><span class="p">(</span><span class="s">'-pub_date'</span><span class="p">)[:</span><span class="mi">5</span><span class="p">]</span>
<span class="k">return</span> <span class="n">render_to_response</span><span class="p">(</span><span class="s">'polls/index.html'</span><span class="p">,</span> <span class="p">{</span><span class="s">'latest_poll_list'</span><span class="p">:</span> <span class="n">latest_poll_list</span><span class="p">})</span>
</pre></div>
</div>
<p>Note that once we’ve done this in all these views, we no longer need to import
<tt class="xref py py-mod docutils literal"><span class="pre">loader</span></tt>, <a class="reference internal" href="../ref/templates/api.html#django.template.Context" title="django.template.Context"><tt class="xref py py-class docutils literal"><span class="pre">Context</span></tt></a> and
<a class="reference internal" href="../ref/request-response.html#django.http.HttpResponse" title="django.http.HttpResponse"><tt class="xref py py-class docutils literal"><span class="pre">HttpResponse</span></tt></a>.</p>
<p>The <a class="reference internal" href="../topics/http/shortcuts.html#django.shortcuts.render_to_response" title="django.shortcuts.render_to_response"><tt class="xref py py-func docutils literal"><span class="pre">render_to_response()</span></tt></a> function takes a template name
as its first argument and a dictionary as its optional second argument. It
returns an <a class="reference internal" href="../ref/request-response.html#django.http.HttpResponse" title="django.http.HttpResponse"><tt class="xref py py-class docutils literal"><span class="pre">HttpResponse</span></tt></a> object of the given template
rendered with the given context.</p>
</div>
</div>
<div class="section" id="s-raising-404">
<span id="raising-404"></span><h2>Raising 404<a class="headerlink" href="#raising-404" title="Permalink to this headline">¶</a></h2>
<p>Now, let’s tackle the poll detail view – the page that displays the question
for a given poll. Here’s the view:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">django.http</span> <span class="kn">import</span> <span class="n">Http404</span>
<span class="c"># ...</span>
<span class="k">def</span> <span class="nf">detail</span><span class="p">(</span><span class="n">request</span><span class="p">,</span> <span class="n">poll_id</span><span class="p">):</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">p</span> <span class="o">=</span> <span class="n">Poll</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">pk</span><span class="o">=</span><span class="n">poll_id</span><span class="p">)</span>
<span class="k">except</span> <span class="n">Poll</span><span class="o">.</span><span class="n">DoesNotExist</span><span class="p">:</span>
<span class="k">raise</span> <span class="n">Http404</span>
<span class="k">return</span> <span class="n">render_to_response</span><span class="p">(</span><span class="s">'polls/detail.html'</span><span class="p">,</span> <span class="p">{</span><span class="s">'poll'</span><span class="p">:</span> <span class="n">p</span><span class="p">})</span>
</pre></div>
</div>
<p>The new concept here: The view raises the <a class="reference internal" href="../topics/http/views.html#django.http.Http404" title="django.http.Http404"><tt class="xref py py-exc docutils literal"><span class="pre">Http404</span></tt></a> exception
if a poll with the requested ID doesn’t exist.</p>
<p>We’ll discuss what you could put in that <tt class="docutils literal"><span class="pre">polls/detail.html</span></tt> template a bit
later, but if you’d like to quickly get the above example working, just:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="p">{{</span> <span class="n">poll</span> <span class="p">}}</span>
</pre></div>
</div>
<p>will get you started for now.</p>
<div class="section" id="s-a-shortcut-get-object-or-404">
<span id="a-shortcut-get-object-or-404"></span><h3>A shortcut: get_object_or_404()<a class="headerlink" href="#a-shortcut-get-object-or-404" title="Permalink to this headline">¶</a></h3>
<p>It’s a very common idiom to use <a class="reference internal" href="../ref/models/querysets.html#django.db.models.query.QuerySet.get" title="django.db.models.query.QuerySet.get"><tt class="xref py py-meth docutils literal"><span class="pre">get()</span></tt></a>
and raise <a class="reference internal" href="../topics/http/views.html#django.http.Http404" title="django.http.Http404"><tt class="xref py py-exc docutils literal"><span class="pre">Http404</span></tt></a> if the object doesn’t exist. Django
provides a shortcut. Here’s the <tt class="docutils literal"><span class="pre">detail()</span></tt> view, rewritten:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">django.shortcuts</span> <span class="kn">import</span> <span class="n">render_to_response</span><span class="p">,</span> <span class="n">get_object_or_404</span>
<span class="c"># ...</span>
<span class="k">def</span> <span class="nf">detail</span><span class="p">(</span><span class="n">request</span><span class="p">,</span> <span class="n">poll_id</span><span class="p">):</span>
<span class="n">p</span> <span class="o">=</span> <span class="n">get_object_or_404</span><span class="p">(</span><span class="n">Poll</span><span class="p">,</span> <span class="n">pk</span><span class="o">=</span><span class="n">poll_id</span><span class="p">)</span>
<span class="k">return</span> <span class="n">render_to_response</span><span class="p">(</span><span class="s">'polls/detail.html'</span><span class="p">,</span> <span class="p">{</span><span class="s">'poll'</span><span class="p">:</span> <span class="n">p</span><span class="p">})</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="../topics/http/shortcuts.html#django.shortcuts.get_object_or_404" title="django.shortcuts.get_object_or_404"><tt class="xref py py-func docutils literal"><span class="pre">get_object_or_404()</span></tt></a> function takes a Django model
as its first argument and an arbitrary number of keyword arguments, which it
passes to the module’s <a class="reference internal" href="../ref/models/querysets.html#django.db.models.query.QuerySet.get" title="django.db.models.query.QuerySet.get"><tt class="xref py py-meth docutils literal"><span class="pre">get()</span></tt></a> function.
It raises <a class="reference internal" href="../topics/http/views.html#django.http.Http404" title="django.http.Http404"><tt class="xref py py-exc docutils literal"><span class="pre">Http404</span></tt></a> if the object doesn’t exist.</p>
<div class="admonition-philosophy admonition">
<p class="first admonition-title">Philosophy</p>
<p>Why do we use a helper function <a class="reference internal" href="../topics/http/shortcuts.html#django.shortcuts.get_object_or_404" title="django.shortcuts.get_object_or_404"><tt class="xref py py-func docutils literal"><span class="pre">get_object_or_404()</span></tt></a>
instead of automatically catching the
<a class="reference internal" href="../ref/exceptions.html#django.core.exceptions.ObjectDoesNotExist" title="django.core.exceptions.ObjectDoesNotExist"><tt class="xref py py-exc docutils literal"><span class="pre">ObjectDoesNotExist</span></tt></a> exceptions at a higher
level, or having the model API raise <a class="reference internal" href="../topics/http/views.html#django.http.Http404" title="django.http.Http404"><tt class="xref py py-exc docutils literal"><span class="pre">Http404</span></tt></a> instead of
<a class="reference internal" href="../ref/exceptions.html#django.core.exceptions.ObjectDoesNotExist" title="django.core.exceptions.ObjectDoesNotExist"><tt class="xref py py-exc docutils literal"><span class="pre">ObjectDoesNotExist</span></tt></a>?</p>
<p class="last">Because that would couple the model layer to the view layer. One of the
foremost design goals of Django is to maintain loose coupling.</p>
</div>
<p>There’s also a <a class="reference internal" href="../topics/http/shortcuts.html#django.shortcuts.get_list_or_404" title="django.shortcuts.get_list_or_404"><tt class="xref py py-func docutils literal"><span class="pre">get_list_or_404()</span></tt></a> function, which works
just as <a class="reference internal" href="../topics/http/shortcuts.html#django.shortcuts.get_object_or_404" title="django.shortcuts.get_object_or_404"><tt class="xref py py-func docutils literal"><span class="pre">get_object_or_404()</span></tt></a> – except using
<a class="reference internal" href="../ref/models/querysets.html#django.db.models.query.QuerySet.filter" title="django.db.models.query.QuerySet.filter"><tt class="xref py py-meth docutils literal"><span class="pre">filter()</span></tt></a> instead of
<a class="reference internal" href="../ref/models/querysets.html#django.db.models.query.QuerySet.get" title="django.db.models.query.QuerySet.get"><tt class="xref py py-meth docutils literal"><span class="pre">get()</span></tt></a>. It raises
<a class="reference internal" href="../topics/http/views.html#django.http.Http404" title="django.http.Http404"><tt class="xref py py-exc docutils literal"><span class="pre">Http404</span></tt></a> if the list is empty.</p>
</div>
</div>
<div class="section" id="s-write-a-404-page-not-found-view">
<span id="write-a-404-page-not-found-view"></span><h2>Write a 404 (page not found) view<a class="headerlink" href="#write-a-404-page-not-found-view" title="Permalink to this headline">¶</a></h2>
<p>When you raise <a class="reference internal" href="../topics/http/views.html#django.http.Http404" title="django.http.Http404"><tt class="xref py py-exc docutils literal"><span class="pre">Http404</span></tt></a> from within a view, Django
will load a special view devoted to handling 404 errors. It finds it
by looking for the variable <tt class="docutils literal"><span class="pre">handler404</span></tt> in your root URLconf (and
only in your root URLconf; setting <tt class="docutils literal"><span class="pre">handler404</span></tt> anywhere else will
have no effect), 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.</p>
<p>You normally won’t have to bother with writing 404 views. If you don’t set
<tt class="docutils literal"><span class="pre">handler404</span></tt>, the built-in view <tt class="xref py py-func docutils literal"><span class="pre">django.views.defaults.page_not_found()</span></tt>
is used by default. In this case, you still have one obligation: create a
<tt class="docutils literal"><span class="pre">404.html</span></tt> template in the root of your template directory. The default 404
view will use that template for all 404 errors. If <a class="reference internal" href="../ref/settings.html#std:setting-DEBUG"><tt class="xref std std-setting docutils literal"><span class="pre">DEBUG</span></tt></a> is set to
<tt class="docutils literal"><span class="pre">False</span></tt> (in your settings module) and if you didn’t create a <tt class="docutils literal"><span class="pre">404.html</span></tt>
file, an <tt class="docutils literal"><span class="pre">Http500</span></tt> is raised instead. So remember to create a <tt class="docutils literal"><span class="pre">404.html</span></tt>.</p>
<p>A couple more things to note about 404 views:</p>
<ul class="simple">
<li>If <a class="reference internal" href="../ref/settings.html#std:setting-DEBUG"><tt class="xref std std-setting docutils literal"><span class="pre">DEBUG</span></tt></a> is set to <tt class="docutils literal"><span class="pre">True</span></tt> (in your settings module) then your
404 view will never be used (and thus the <tt class="docutils literal"><span class="pre">404.html</span></tt> template will never
be rendered) because the traceback will be displayed instead.</li>
<li>The 404 view is also called if Django doesn’t find a match after checking
every regular expression in the URLconf.</li>
</ul>
</div>
<div class="section" id="s-write-a-500-server-error-view">
<span id="write-a-500-server-error-view"></span><h2>Write a 500 (server error) view<a class="headerlink" href="#write-a-500-server-error-view" title="Permalink to this headline">¶</a></h2>
<p>Similarly, your root URLconf may define a <tt class="docutils literal"><span class="pre">handler500</span></tt>, which points
to a view to call in case of server errors. Server errors happen when
you have runtime errors in view code.</p>
</div>
<div class="section" id="s-use-the-template-system">
<span id="use-the-template-system"></span><h2>Use the template system<a class="headerlink" href="#use-the-template-system" title="Permalink to this headline">¶</a></h2>
<p>Back to the <tt class="docutils literal"><span class="pre">detail()</span></tt> view for our poll application. Given the context
variable <tt class="docutils literal"><span class="pre">poll</span></tt>, here’s what the “polls/detail.html” template might look
like:</p>
<div class="highlight-html+django"><div class="highlight"><pre><span class="nt"><h1></span><span class="cp">{{</span> <span class="nv">poll.question</span> <span class="cp">}}</span><span class="nt"></h1></span>
<span class="nt"><ul></span>
<span class="cp">{%</span> <span class="k">for</span> <span class="nv">choice</span> <span class="k">in</span> <span class="nv">poll.choice_set.all</span> <span class="cp">%}</span>
<span class="nt"><li></span><span class="cp">{{</span> <span class="nv">choice.choice</span> <span class="cp">}}</span><span class="nt"></li></span>
<span class="cp">{%</span> <span class="k">endfor</span> <span class="cp">%}</span>
<span class="nt"></ul></span>
</pre></div>
</div>
<p>The template system uses dot-lookup syntax to access variable attributes. In
the example of <tt class="docutils literal"><span class="pre">{{</span> <span class="pre">poll.question</span> <span class="pre">}}</span></tt>, first Django does a dictionary lookup
on the object <tt class="docutils literal"><span class="pre">poll</span></tt>. 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.</p>
<p>Method-calling happens in the <a class="reference internal" href="../ref/templates/builtins.html#std:templatetag-for"><tt class="xref std std-ttag docutils literal"><span class="pre">{%</span> <span class="pre">for</span> <span class="pre">%}</span></tt></a> loop:
<tt class="docutils literal"><span class="pre">poll.choice_set.all</span></tt> is interpreted as the Python code
<tt class="docutils literal"><span class="pre">poll.choice_set.all()</span></tt>, which returns an iterable of Choice objects and is
suitable for use in the <a class="reference internal" href="../ref/templates/builtins.html#std:templatetag-for"><tt class="xref std std-ttag docutils literal"><span class="pre">{%</span> <span class="pre">for</span> <span class="pre">%}</span></tt></a> tag.</p>
<p>See the <a class="reference internal" href="../topics/templates.html"><em>template guide</em></a> for more about templates.</p>
</div>
<div class="section" id="s-simplifying-the-urlconfs">
<span id="simplifying-the-urlconfs"></span><h2>Simplifying the URLconfs<a class="headerlink" href="#simplifying-the-urlconfs" title="Permalink to this headline">¶</a></h2>
<p>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:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">urlpatterns</span> <span class="o">=</span> <span class="n">patterns</span><span class="p">(</span><span class="s">''</span><span class="p">,</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/$'</span><span class="p">,</span> <span class="s">'polls.views.index'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/(?P<poll_id>\d+)/$'</span><span class="p">,</span> <span class="s">'polls.views.detail'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/(?P<poll_id>\d+)/results/$'</span><span class="p">,</span> <span class="s">'polls.views.results'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/(?P<poll_id>\d+)/vote/$'</span><span class="p">,</span> <span class="s">'polls.views.vote'</span><span class="p">),</span>
<span class="p">)</span>
</pre></div>
</div>
<p>Namely, <tt class="docutils literal"><span class="pre">polls.views</span></tt> is in every callback.</p>
<p>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 <a class="reference internal" href="../topics/http/urls.html#django.conf.urls.patterns" title="django.conf.urls.patterns"><tt class="xref py py-func docutils literal"><span class="pre">patterns()</span></tt></a>, like so:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">urlpatterns</span> <span class="o">=</span> <span class="n">patterns</span><span class="p">(</span><span class="s">'polls.views'</span><span class="p">,</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/$'</span><span class="p">,</span> <span class="s">'index'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/(?P<poll_id>\d+)/$'</span><span class="p">,</span> <span class="s">'detail'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/(?P<poll_id>\d+)/results/$'</span><span class="p">,</span> <span class="s">'results'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/(?P<poll_id>\d+)/vote/$'</span><span class="p">,</span> <span class="s">'vote'</span><span class="p">),</span>
<span class="p">)</span>
</pre></div>
</div>
<p>This is functionally identical to the previous formatting. It’s just a bit
tidier.</p>
<p>Since you generally don’t want the prefix for one app to be applied to every
callback in your URLconf, you can concatenate multiple
<a class="reference internal" href="../topics/http/urls.html#django.conf.urls.patterns" title="django.conf.urls.patterns"><tt class="xref py py-func docutils literal"><span class="pre">patterns()</span></tt></a>. Your full <tt class="docutils literal"><span class="pre">mysite/urls.py</span></tt> might
now look like this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">django.conf.urls</span> <span class="kn">import</span> <span class="n">patterns</span><span class="p">,</span> <span class="n">include</span><span class="p">,</span> <span class="n">url</span>
<span class="kn">from</span> <span class="nn">django.contrib</span> <span class="kn">import</span> <span class="n">admin</span>
<span class="n">admin</span><span class="o">.</span><span class="n">autodiscover</span><span class="p">()</span>
<span class="n">urlpatterns</span> <span class="o">=</span> <span class="n">patterns</span><span class="p">(</span><span class="s">'polls.views'</span><span class="p">,</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/$'</span><span class="p">,</span> <span class="s">'index'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/(?P<poll_id>\d+)/$'</span><span class="p">,</span> <span class="s">'detail'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/(?P<poll_id>\d+)/results/$'</span><span class="p">,</span> <span class="s">'results'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/(?P<poll_id>\d+)/vote/$'</span><span class="p">,</span> <span class="s">'vote'</span><span class="p">),</span>
<span class="p">)</span>
<span class="n">urlpatterns</span> <span class="o">+=</span> <span class="n">patterns</span><span class="p">(</span><span class="s">''</span><span class="p">,</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^admin/'</span><span class="p">,</span> <span class="n">include</span><span class="p">(</span><span class="n">admin</span><span class="o">.</span><span class="n">site</span><span class="o">.</span><span class="n">urls</span><span class="p">)),</span>
<span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="s-decoupling-the-urlconfs">
<span id="decoupling-the-urlconfs"></span><h2>Decoupling the URLconfs<a class="headerlink" href="#decoupling-the-urlconfs" title="Permalink to this headline">¶</a></h2>
<p>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.</p>
<p>Our poll app is pretty decoupled at this point, thanks to the strict directory
structure that <tt class="docutils literal"><span class="pre">python</span> <span class="pre">manage.py</span> <span class="pre">startapp</span></tt> created, but one part of it is
coupled to the Django settings: The URLconf.</p>
<p>We’ve been editing the URLs in <tt class="docutils literal"><span class="pre">mysite/urls.py</span></tt>, 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.</p>
<p>Copy the file <tt class="docutils literal"><span class="pre">mysite/urls.py</span></tt> to <tt class="docutils literal"><span class="pre">polls/urls.py</span></tt>. Then, change
<tt class="docutils literal"><span class="pre">mysite/urls.py</span></tt> to remove the poll-specific URLs and insert an
<a class="reference internal" href="../topics/http/urls.html#django.conf.urls.include" title="django.conf.urls.include"><tt class="xref py py-func docutils literal"><span class="pre">include()</span></tt></a>, leaving you with:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">django.conf.urls</span> <span class="kn">import</span> <span class="n">patterns</span><span class="p">,</span> <span class="n">include</span><span class="p">,</span> <span class="n">url</span>
<span class="kn">from</span> <span class="nn">django.contrib</span> <span class="kn">import</span> <span class="n">admin</span>
<span class="n">admin</span><span class="o">.</span><span class="n">autodiscover</span><span class="p">()</span>
<span class="n">urlpatterns</span> <span class="o">=</span> <span class="n">patterns</span><span class="p">(</span><span class="s">''</span><span class="p">,</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^polls/'</span><span class="p">,</span> <span class="n">include</span><span class="p">(</span><span class="s">'polls.urls'</span><span class="p">)),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^admin/'</span><span class="p">,</span> <span class="n">include</span><span class="p">(</span><span class="n">admin</span><span class="o">.</span><span class="n">site</span><span class="o">.</span><span class="n">urls</span><span class="p">)),</span>
<span class="p">)</span>
</pre></div>
</div>
<p><a class="reference internal" href="../topics/http/urls.html#django.conf.urls.include" title="django.conf.urls.include"><tt class="xref py py-func docutils literal"><span class="pre">include()</span></tt></a> simply references another URLconf.
Note that the regular expression doesn’t have a <tt class="docutils literal"><span class="pre">$</span></tt> (end-of-string match
character) but has the trailing slash. Whenever Django encounters
<a class="reference internal" href="../topics/http/urls.html#django.conf.urls.include" title="django.conf.urls.include"><tt class="xref py py-func docutils literal"><span class="pre">include()</span></tt></a>, it chops off whatever part of the
URL matched up to that point and sends the remaining string to the included
URLconf for further processing.</p>
<p>Here’s what happens if a user goes to “/polls/34/” in this system:</p>
<ul class="simple">
<li>Django will find the match at <tt class="docutils literal"><span class="pre">'^polls/'</span></tt></li>
<li>Then, Django will strip off the matching text (<tt class="docutils literal"><span class="pre">"polls/"</span></tt>) and send the
remaining text – <tt class="docutils literal"><span class="pre">"34/"</span></tt> – to the ‘polls.urls’ URLconf for
further processing.</li>
</ul>
<p>Now that we’ve decoupled that, we need to decouple the <tt class="docutils literal"><span class="pre">polls.urls</span></tt>
URLconf by removing the leading “polls/” from each line, and removing the
lines registering the admin site. Your <tt class="docutils literal"><span class="pre">polls/urls.py</span></tt> file should now look like
this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">django.conf.urls</span> <span class="kn">import</span> <span class="n">patterns</span><span class="p">,</span> <span class="n">url</span>
<span class="n">urlpatterns</span> <span class="o">=</span> <span class="n">patterns</span><span class="p">(</span><span class="s">'polls.views'</span><span class="p">,</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^$'</span><span class="p">,</span> <span class="s">'index'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^(?P<poll_id>\d+)/$'</span><span class="p">,</span> <span class="s">'detail'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^(?P<poll_id>\d+)/results/$'</span><span class="p">,</span> <span class="s">'results'</span><span class="p">),</span>
<span class="n">url</span><span class="p">(</span><span class="s">r'^(?P<poll_id>\d+)/vote/$'</span><span class="p">,</span> <span class="s">'vote'</span><span class="p">),</span>
<span class="p">)</span>
</pre></div>
</div>
<p>The idea behind <a class="reference internal" href="../topics/http/urls.html#django.conf.urls.include" title="django.conf.urls.include"><tt class="xref py py-func docutils literal"><span class="pre">include()</span></tt></a> 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.</p>
<p>All the poll app cares about is its relative path, not its absolute path.</p>
<p>When you’re comfortable with writing views, read <a class="reference internal" href="tutorial04.html"><em>part 4 of this tutorial</em></a> to learn about simple form processing and generic views.</p>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Writing your first Django app, part 3</a><ul>
<li><a class="reference internal" href="#philosophy">Philosophy</a></li>
<li><a class="reference internal" href="#design-your-urls">Design your URLs</a></li>
<li><a class="reference internal" href="#write-your-first-view">Write your first view</a></li>
<li><a class="reference internal" href="#write-views-that-actually-do-something">Write views that actually do something</a><ul>
<li><a class="reference internal" href="#a-shortcut-render-to-response">A shortcut: render_to_response()</a></li>
</ul>
</li>
<li><a class="reference internal" href="#raising-404">Raising 404</a><ul>
<li><a class="reference internal" href="#a-shortcut-get-object-or-404">A shortcut: get_object_or_404()</a></li>
</ul>
</li>
<li><a class="reference internal" href="#write-a-404-page-not-found-view">Write a 404 (page not found) view</a></li>
<li><a class="reference internal" href="#write-a-500-server-error-view">Write a 500 (server error) view</a></li>
<li><a class="reference internal" href="#use-the-template-system">Use the template system</a></li>
<li><a class="reference internal" href="#simplifying-the-urlconfs">Simplifying the URLconfs</a></li>
<li><a class="reference internal" href="#decoupling-the-urlconfs">Decoupling the URLconfs</a></li>
</ul>
</li>
</ul>
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