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<h1><a href="../index.html">Django 1.4.5 documentation</a></h1>
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<div class="section" id="s-module-django.dispatch">
<span id="s-signals"></span><span id="module-django.dispatch"></span><span id="signals"></span><h1>Signals<a class="headerlink" href="#module-django.dispatch" title="Permalink to this headline">¶</a></h1>
<p>Django includes a “signal dispatcher” which helps allow decoupled applications
get notified when actions occur elsewhere in the framework. In a nutshell,
signals allow certain <em>senders</em> to notify a set of <em>receivers</em> that some action
has taken place. They’re especially useful when many pieces of code may be
interested in the same events.</p>
<p>Django provides a <a class="reference internal" href="../ref/signals.html"><em>set of built-in signals</em></a> that let user
code get notified by Django itself of certain actions. These include some useful
notifications:</p>
<ul>
<li><p class="first"><a class="reference internal" href="../ref/signals.html#django.db.models.signals.pre_save" title="django.db.models.signals.pre_save"><tt class="xref py py-data docutils literal"><span class="pre">django.db.models.signals.pre_save</span></tt></a> &
<a class="reference internal" href="../ref/signals.html#django.db.models.signals.post_save" title="django.db.models.signals.post_save"><tt class="xref py py-data docutils literal"><span class="pre">django.db.models.signals.post_save</span></tt></a></p>
<p>Sent before or after a model’s <a class="reference internal" href="../ref/models/instances.html#django.db.models.Model.save" title="django.db.models.Model.save"><tt class="xref py py-meth docutils literal"><span class="pre">save()</span></tt></a> method
is called.</p>
</li>
<li><p class="first"><a class="reference internal" href="../ref/signals.html#django.db.models.signals.pre_delete" title="django.db.models.signals.pre_delete"><tt class="xref py py-data docutils literal"><span class="pre">django.db.models.signals.pre_delete</span></tt></a> &
<a class="reference internal" href="../ref/signals.html#django.db.models.signals.post_delete" title="django.db.models.signals.post_delete"><tt class="xref py py-data docutils literal"><span class="pre">django.db.models.signals.post_delete</span></tt></a></p>
<p>Sent before or after a model’s <a class="reference internal" href="../ref/models/instances.html#django.db.models.Model.delete" title="django.db.models.Model.delete"><tt class="xref py py-meth docutils literal"><span class="pre">delete()</span></tt></a>
method or queryset’s <a class="reference internal" href="../ref/models/querysets.html#django.db.models.query.QuerySet.delete" title="django.db.models.query.QuerySet.delete"><tt class="xref py py-meth docutils literal"><span class="pre">delete()</span></tt></a>
method is called.</p>
</li>
<li><p class="first"><a class="reference internal" href="../ref/signals.html#django.db.models.signals.m2m_changed" title="django.db.models.signals.m2m_changed"><tt class="xref py py-data docutils literal"><span class="pre">django.db.models.signals.m2m_changed</span></tt></a></p>
<p>Sent when a <tt class="xref py py-class docutils literal"><span class="pre">ManyToManyField</span></tt> on a model is changed.</p>
</li>
<li><p class="first"><a class="reference internal" href="../ref/signals.html#django.core.signals.request_started" title="django.core.signals.request_started"><tt class="xref py py-data docutils literal"><span class="pre">django.core.signals.request_started</span></tt></a> &
<a class="reference internal" href="../ref/signals.html#django.core.signals.request_finished" title="django.core.signals.request_finished"><tt class="xref py py-data docutils literal"><span class="pre">django.core.signals.request_finished</span></tt></a></p>
<p>Sent when Django starts or finishes an HTTP request.</p>
</li>
</ul>
<p>See the <a class="reference internal" href="../ref/signals.html"><em>built-in signal documentation</em></a> for a complete list,
and a complete explanation of each signal.</p>
<p>You can also <a class="reference internal" href="#defining-and-sending-signals">define and send your own custom signals</a>; see below.</p>
<div class="section" id="s-listening-to-signals">
<span id="listening-to-signals"></span><h2>Listening to signals<a class="headerlink" href="#listening-to-signals" title="Permalink to this headline">¶</a></h2>
<p>To receive a signal, you need to register a <em>receiver</em> function that gets
called when the signal is sent by using the
<a class="reference internal" href="#django.dispatch.Signal.connect" title="django.dispatch.Signal.connect"><tt class="xref py py-meth docutils literal"><span class="pre">Signal.connect()</span></tt></a> method:</p>
<dl class="method">
<dt id="django.dispatch.Signal.connect">
<tt class="descclassname">Signal.</tt><tt class="descname">connect</tt>(<em>receiver</em><span class="optional">[</span>, <em>sender=None</em>, <em>weak=True</em>, <em>dispatch_uid=None</em><span class="optional">]</span>)<a class="headerlink" href="#django.dispatch.Signal.connect" title="Permalink to this definition">¶</a></dt>
<dd><table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first last simple">
<li><strong>receiver</strong> – The callback function which will be connected to this
signal. See <a class="reference internal" href="#receiver-functions"><em>Receiver functions</em></a> for more information.</li>
<li><strong>sender</strong> – Specifies a particular sender to receive signals from. See
<a class="reference internal" href="#connecting-to-specific-signals"><em>Connecting to signals sent by specific senders</em></a> for more information.</li>
<li><strong>weak</strong> – Django stores signal handlers as weak references by
default. Thus, if your receiver is a local function, it may be
garbage collected. To prevent this, pass <tt class="docutils literal"><span class="pre">weak=False</span></tt> when you call
the signal’s <tt class="docutils literal"><span class="pre">connect()</span></tt> method.</li>
<li><strong>dispatch_uid</strong> – A unique identifier for a signal receiver in cases
where duplicate signals may be sent. See
<a class="reference internal" href="#preventing-duplicate-signals"><em>Preventing duplicate signals</em></a> for more information.</li>
</ul>
</td>
</tr>
</tbody>
</table>
</dd></dl>
<p>Let’s see how this works by registering a signal that
gets called after each HTTP request is finished. We’ll be connecting to the
<a class="reference internal" href="../ref/signals.html#django.core.signals.request_finished" title="django.core.signals.request_finished"><tt class="xref py py-data docutils literal"><span class="pre">request_finished</span></tt></a> signal.</p>
<div class="section" id="s-receiver-functions">
<span id="s-id1"></span><span id="receiver-functions"></span><span id="id1"></span><h3>Receiver functions<a class="headerlink" href="#receiver-functions" title="Permalink to this headline">¶</a></h3>
<p>First, we need to define a receiver function. A receiver can be any Python
function or method:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">my_callback</span><span class="p">(</span><span class="n">sender</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="k">print</span> <span class="s">"Request finished!"</span>
</pre></div>
</div>
<p>Notice that the function takes a <tt class="docutils literal"><span class="pre">sender</span></tt> argument, along with wildcard
keyword arguments (<tt class="docutils literal"><span class="pre">**kwargs</span></tt>); all signal handlers must take these arguments.</p>
<p>We’ll look at senders <a class="reference internal" href="#connecting-to-signals-sent-by-specific-senders">a bit later</a>, but right now look at the <tt class="docutils literal"><span class="pre">**kwargs</span></tt>
argument. All signals send keyword arguments, and may change those keyword
arguments at any time. In the case of
<a class="reference internal" href="../ref/signals.html#django.core.signals.request_finished" title="django.core.signals.request_finished"><tt class="xref py py-data docutils literal"><span class="pre">request_finished</span></tt></a>, it’s documented as sending no
arguments, which means we might be tempted to write our signal handling as
<tt class="docutils literal"><span class="pre">my_callback(sender)</span></tt>.</p>
<p>This would be wrong – in fact, Django will throw an error if you do so. That’s
because at any point arguments could get added to the signal and your receiver
must be able to handle those new arguments.</p>
</div>
<div class="section" id="s-connecting-receiver-functions">
<span id="s-id2"></span><span id="connecting-receiver-functions"></span><span id="id2"></span><h3>Connecting receiver functions<a class="headerlink" href="#connecting-receiver-functions" title="Permalink to this headline">¶</a></h3>
<p>There are two ways you can connect a receiver to a signal. You can take the
manual connect route:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">django.core.signals</span> <span class="kn">import</span> <span class="n">request_finished</span>
<span class="n">request_finished</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="n">my_callback</span><span class="p">)</span>
</pre></div>
</div>
<p>Alternatively, you can use a <tt class="docutils literal"><span class="pre">receiver</span></tt> decorator when you define your
receiver:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">django.core.signals</span> <span class="kn">import</span> <span class="n">request_finished</span>
<span class="kn">from</span> <span class="nn">django.dispatch</span> <span class="kn">import</span> <span class="n">receiver</span>
<span class="nd">@receiver</span><span class="p">(</span><span class="n">request_finished</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">my_callback</span><span class="p">(</span><span class="n">sender</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="k">print</span> <span class="s">"Request finished!"</span>
</pre></div>
</div>
<p>Now, our <tt class="docutils literal"><span class="pre">my_callback</span></tt> function will be called each time a request finishes.</p>
<div class="versionadded">
<span class="title">New in Django 1.3:</span> <a class="reference internal" href="../releases/1.3.html"><em>Please see the release notes</em></a></div>
<p>The <tt class="docutils literal"><span class="pre">receiver</span></tt> decorator was added in Django 1.3.</p>
<div class="admonition-where-should-this-code-live admonition">
<p class="first admonition-title">Where should this code live?</p>
<p class="last">You can put signal handling and registration code anywhere you like.
However, you’ll need to make sure that the module it’s in gets imported
early on so that the signal handling gets registered before any signals need
to be sent. This makes your app’s <tt class="docutils literal"><span class="pre">models.py</span></tt> a good place to put
registration of signal handlers.</p>
</div>
</div>
<div class="section" id="s-connecting-to-signals-sent-by-specific-senders">
<span id="s-connecting-to-specific-signals"></span><span id="connecting-to-signals-sent-by-specific-senders"></span><span id="connecting-to-specific-signals"></span><h3>Connecting to signals sent by specific senders<a class="headerlink" href="#connecting-to-signals-sent-by-specific-senders" title="Permalink to this headline">¶</a></h3>
<p>Some signals get sent many times, but you’ll only be interested in receiving a
certain subset of those signals. For example, consider the
<a class="reference internal" href="../ref/signals.html#django.db.models.signals.pre_save" title="django.db.models.signals.pre_save"><tt class="xref py py-data docutils literal"><span class="pre">django.db.models.signals.pre_save</span></tt></a> signal sent before a model gets saved.
Most of the time, you don’t need to know when <em>any</em> model gets saved – just
when one <em>specific</em> model is saved.</p>
<p>In these cases, you can register to receive signals sent only by particular
senders. In the case of <a class="reference internal" href="../ref/signals.html#django.db.models.signals.pre_save" title="django.db.models.signals.pre_save"><tt class="xref py py-data docutils literal"><span class="pre">django.db.models.signals.pre_save</span></tt></a>, the sender
will be the model class being saved, so you can indicate that you only want
signals sent by some model:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">django.db.models.signals</span> <span class="kn">import</span> <span class="n">pre_save</span>
<span class="kn">from</span> <span class="nn">django.dispatch</span> <span class="kn">import</span> <span class="n">receiver</span>
<span class="kn">from</span> <span class="nn">myapp.models</span> <span class="kn">import</span> <span class="n">MyModel</span>
<span class="nd">@receiver</span><span class="p">(</span><span class="n">pre_save</span><span class="p">,</span> <span class="n">sender</span><span class="o">=</span><span class="n">MyModel</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">my_handler</span><span class="p">(</span><span class="n">sender</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="o">...</span>
</pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">my_handler</span></tt> function will only be called when an instance of <tt class="docutils literal"><span class="pre">MyModel</span></tt>
is saved.</p>
<p>Different signals use different objects as their senders; you’ll need to consult
the <a class="reference internal" href="../ref/signals.html"><em>built-in signal documentation</em></a> for details of each
particular signal.</p>
</div>
<div class="section" id="s-preventing-duplicate-signals">
<span id="s-id3"></span><span id="preventing-duplicate-signals"></span><span id="id3"></span><h3>Preventing duplicate signals<a class="headerlink" href="#preventing-duplicate-signals" title="Permalink to this headline">¶</a></h3>
<p>In some circumstances, the module in which you are connecting signals may be
imported multiple times. This can cause your receiver function to be
registered more than once, and thus called multiples times for a single signal
event.</p>
<p>If this behavior is problematic (such as when using signals to
send an email whenever a model is saved), pass a unique identifier as
the <tt class="docutils literal"><span class="pre">dispatch_uid</span></tt> argument to identify your receiver function. This
identifier will usually be a string, although any hashable object will
suffice. The end result is that your receiver function will only be
bound to the signal once for each unique <tt class="docutils literal"><span class="pre">dispatch_uid</span></tt> value.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">django.core.signals</span> <span class="kn">import</span> <span class="n">request_finished</span>
<span class="n">request_finished</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="n">my_callback</span><span class="p">,</span> <span class="n">dispatch_uid</span><span class="o">=</span><span class="s">"my_unique_identifier"</span><span class="p">)</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="s-defining-and-sending-signals">
<span id="defining-and-sending-signals"></span><h2>Defining and sending signals<a class="headerlink" href="#defining-and-sending-signals" title="Permalink to this headline">¶</a></h2>
<p>Your applications can take advantage of the signal infrastructure and provide
its own signals.</p>
<div class="section" id="s-defining-signals">
<span id="defining-signals"></span><h3>Defining signals<a class="headerlink" href="#defining-signals" title="Permalink to this headline">¶</a></h3>
<dl class="class">
<dt id="django.dispatch.Signal">
<em class="property">class </em><tt class="descname">Signal</tt>(<span class="optional">[</span><em>providing_args=list</em><span class="optional">]</span>)<a class="headerlink" href="#django.dispatch.Signal" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>All signals are <a class="reference internal" href="#django.dispatch.Signal" title="django.dispatch.Signal"><tt class="xref py py-class docutils literal"><span class="pre">django.dispatch.Signal</span></tt></a> instances. The
<tt class="docutils literal"><span class="pre">providing_args</span></tt> is a list of the names of arguments the signal will provide
to listeners.</p>
<p>For example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">django.dispatch</span>
<span class="n">pizza_done</span> <span class="o">=</span> <span class="n">django</span><span class="o">.</span><span class="n">dispatch</span><span class="o">.</span><span class="n">Signal</span><span class="p">(</span><span class="n">providing_args</span><span class="o">=</span><span class="p">[</span><span class="s">"toppings"</span><span class="p">,</span> <span class="s">"size"</span><span class="p">])</span>
</pre></div>
</div>
<p>This declares a <tt class="docutils literal"><span class="pre">pizza_done</span></tt> signal that will provide receivers with
<tt class="docutils literal"><span class="pre">toppings</span></tt> and <tt class="docutils literal"><span class="pre">size</span></tt> arguments.</p>
<p>Remember that you’re allowed to change this list of arguments at any time, so getting the API right on the first try isn’t necessary.</p>
</div>
<div class="section" id="s-sending-signals">
<span id="sending-signals"></span><h3>Sending signals<a class="headerlink" href="#sending-signals" title="Permalink to this headline">¶</a></h3>
<p>There are two ways to send send signals in Django.</p>
<dl class="method">
<dt id="django.dispatch.Signal.send">
<tt class="descclassname">Signal.</tt><tt class="descname">send</tt>(<em>sender</em>, <em>**kwargs</em>)<a class="headerlink" href="#django.dispatch.Signal.send" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<dl class="method">
<dt id="django.dispatch.Signal.send_robust">
<tt class="descclassname">Signal.</tt><tt class="descname">send_robust</tt>(<em>sender</em>, <em>**kwargs</em>)<a class="headerlink" href="#django.dispatch.Signal.send_robust" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>To send a signal, call either <a class="reference internal" href="#django.dispatch.Signal.send" title="django.dispatch.Signal.send"><tt class="xref py py-meth docutils literal"><span class="pre">Signal.send()</span></tt></a> or <a class="reference internal" href="#django.dispatch.Signal.send_robust" title="django.dispatch.Signal.send_robust"><tt class="xref py py-meth docutils literal"><span class="pre">Signal.send_robust()</span></tt></a>.
You must provide the <tt class="docutils literal"><span class="pre">sender</span></tt> argument, and may provide as many other keyword
arguments as you like.</p>
<p>For example, here’s how sending our <tt class="docutils literal"><span class="pre">pizza_done</span></tt> signal might look:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">PizzaStore</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="o">...</span>
<span class="k">def</span> <span class="nf">send_pizza</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">toppings</span><span class="p">,</span> <span class="n">size</span><span class="p">):</span>
<span class="n">pizza_done</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="n">sender</span><span class="o">=</span><span class="bp">self</span><span class="p">,</span> <span class="n">toppings</span><span class="o">=</span><span class="n">toppings</span><span class="p">,</span> <span class="n">size</span><span class="o">=</span><span class="n">size</span><span class="p">)</span>
<span class="o">...</span>
</pre></div>
</div>
<p>Both <tt class="docutils literal"><span class="pre">send()</span></tt> and <tt class="docutils literal"><span class="pre">send_robust()</span></tt> return a list of tuple pairs
<tt class="docutils literal"><span class="pre">[(receiver,</span> <span class="pre">response),</span> <span class="pre">...</span> <span class="pre">]</span></tt>, representing the list of called receiver
functions and their response values.</p>
<p><tt class="docutils literal"><span class="pre">send()</span></tt> differs from <tt class="docutils literal"><span class="pre">send_robust()</span></tt> in how exceptions raised by receiver
functions are handled. <tt class="docutils literal"><span class="pre">send()</span></tt> does <em>not</em> catch any exceptions raised by
receivers; it simply allows errors to propagate. Thus not all receivers may
be notified of a signal in the face of an error.</p>
<p><tt class="docutils literal"><span class="pre">send_robust()</span></tt> catches all errors derived from Python’s <tt class="docutils literal"><span class="pre">Exception</span></tt> class,
and ensures all receivers are notified of the signal. If an error occurs, the
error instance is returned in the tuple pair for the receiver that raised the error.</p>
</div>
</div>
<div class="section" id="s-disconnecting-signals">
<span id="disconnecting-signals"></span><h2>Disconnecting signals<a class="headerlink" href="#disconnecting-signals" title="Permalink to this headline">¶</a></h2>
<dl class="method">
<dt id="django.dispatch.Signal.disconnect">
<tt class="descclassname">Signal.</tt><tt class="descname">disconnect</tt>(<span class="optional">[</span><em>receiver=None</em>, <em>sender=None</em>, <em>weak=True</em>, <em>dispatch_uid=None</em><span class="optional">]</span>)<a class="headerlink" href="#django.dispatch.Signal.disconnect" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>To disconnect a receiver from a signal, call <a class="reference internal" href="#django.dispatch.Signal.disconnect" title="django.dispatch.Signal.disconnect"><tt class="xref py py-meth docutils literal"><span class="pre">Signal.disconnect()</span></tt></a>. The
arguments are as described in <a class="reference internal" href="#django.dispatch.Signal.connect" title="django.dispatch.Signal.connect"><tt class="xref py py-meth docutils literal"><span class="pre">Signal.connect()</span></tt></a>.</p>
<p>The <em>receiver</em> argument indicates the registered receiver to disconnect. It may
be <tt class="docutils literal"><span class="pre">None</span></tt> if <tt class="docutils literal"><span class="pre">dispatch_uid</span></tt> is used to identify the receiver.</p>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
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<li><a class="reference internal" href="#">Signals</a><ul>
<li><a class="reference internal" href="#listening-to-signals">Listening to signals</a><ul>
<li><a class="reference internal" href="#receiver-functions">Receiver functions</a></li>
<li><a class="reference internal" href="#connecting-receiver-functions">Connecting receiver functions</a></li>
<li><a class="reference internal" href="#connecting-to-signals-sent-by-specific-senders">Connecting to signals sent by specific senders</a></li>
<li><a class="reference internal" href="#preventing-duplicate-signals">Preventing duplicate signals</a></li>
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<li><a class="reference internal" href="#defining-and-sending-signals">Defining and sending signals</a><ul>
<li><a class="reference internal" href="#defining-signals">Defining signals</a></li>
<li><a class="reference internal" href="#sending-signals">Sending signals</a></li>
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<li><a class="reference internal" href="#disconnecting-signals">Disconnecting signals</a></li>
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