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  <div class="section" id="signals">
<h1>Signals<a class="headerlink" href="#signals" title="Permalink to this headline"></a></h1>
<div class="section" id="decoupling-with-named-signals">
<h2>Decoupling With Named Signals<a class="headerlink" href="#decoupling-with-named-signals" title="Permalink to this headline"></a></h2>
<p>Named signals are created with <a class="reference internal" href="api.html#blinker.base.signal" title="blinker.base.signal"><tt class="xref py py-func docutils literal"><span class="pre">signal()</span></tt></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">blinker</span> <span class="kn">import</span> <span class="n">signal</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">initialized</span> <span class="o">=</span> <span class="n">signal</span><span class="p">(</span><span class="s">&#39;initialized&#39;</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">initialized</span> <span class="ow">is</span> <span class="n">signal</span><span class="p">(</span><span class="s">&#39;initialized&#39;</span><span class="p">)</span>
<span class="go">True</span>
</pre></div>
</div>
<p>Every call to <tt class="docutils literal"><span class="pre">signal('name')</span></tt> returns the same signal object,
allowing unconnected parts of code (different modules, plugins,
anything) to all use the same signal without requiring any code
sharing or special imports.</p>
</div>
<div class="section" id="subscribing-to-signals">
<h2>Subscribing to Signals<a class="headerlink" href="#subscribing-to-signals" title="Permalink to this headline"></a></h2>
<p><a class="reference internal" href="api.html#blinker.base.Signal.connect" title="blinker.base.Signal.connect"><tt class="xref py py-meth docutils literal"><span class="pre">Signal.connect()</span></tt></a> registers a function to be invoked each time
the signal is emitted.  Connected functions are always passed the
object that caused the signal to be emitted.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="k">def</span> <span class="nf">subscriber</span><span class="p">(</span><span class="n">sender</span><span class="p">):</span>
<span class="gp">... </span>    <span class="k">print</span><span class="p">(</span><span class="s">&quot;Got a signal sent by </span><span class="si">%r</span><span class="s">&quot;</span> <span class="o">%</span> <span class="n">sender</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">ready</span> <span class="o">=</span> <span class="n">signal</span><span class="p">(</span><span class="s">&#39;ready&#39;</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">ready</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="n">subscriber</span><span class="p">)</span>
<span class="go">&lt;function subscriber at 0x...&gt;</span>
</pre></div>
</div>
</div>
<div class="section" id="emitting-signals">
<h2>Emitting Signals<a class="headerlink" href="#emitting-signals" title="Permalink to this headline"></a></h2>
<p>Code producing events of interest can <a class="reference internal" href="api.html#blinker.base.Signal.send" title="blinker.base.Signal.send"><tt class="xref py py-meth docutils literal"><span class="pre">Signal.send()</span></tt></a>
notifications to all connected receivers.</p>
<p>Below, a simple <tt class="docutils literal"><span class="pre">Processor</span></tt> class emits a <tt class="docutils literal"><span class="pre">ready</span></tt> signal when it&#8217;s
about to process something, and <tt class="docutils literal"><span class="pre">complete</span></tt> when it is done.  It
passes <tt class="docutils literal"><span class="pre">self</span></tt> to the <a class="reference internal" href="api.html#blinker.base.Signal.send" title="blinker.base.Signal.send"><tt class="xref py py-meth docutils literal"><span class="pre">send()</span></tt></a> method, signifying that
that particular instance was responsible for emitting the signal.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">Processor</span><span class="p">:</span>
<span class="gp">... </span>   <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>
<span class="gp">... </span>       <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
<span class="gp">...</span>
<span class="gp">... </span>   <span class="k">def</span> <span class="nf">go</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span>       <span class="n">ready</span> <span class="o">=</span> <span class="n">signal</span><span class="p">(</span><span class="s">&#39;ready&#39;</span><span class="p">)</span>
<span class="gp">... </span>       <span class="n">ready</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span>
<span class="gp">... </span>       <span class="k">print</span><span class="p">(</span><span class="s">&quot;Processing.&quot;</span><span class="p">)</span>
<span class="gp">... </span>       <span class="n">complete</span> <span class="o">=</span> <span class="n">signal</span><span class="p">(</span><span class="s">&#39;complete&#39;</span><span class="p">)</span>
<span class="gp">... </span>       <span class="n">complete</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">... </span>   <span class="k">def</span> <span class="nf">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span>       <span class="k">return</span> <span class="s">&#39;&lt;Processor </span><span class="si">%s</span><span class="s">&gt;&#39;</span> <span class="o">%</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span>
<span class="gp">...</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">processor_a</span> <span class="o">=</span> <span class="n">Processor</span><span class="p">(</span><span class="s">&#39;a&#39;</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">processor_a</span><span class="o">.</span><span class="n">go</span><span class="p">()</span>
<span class="go">Got a signal sent by &lt;Processor a&gt;</span>
<span class="go">Processing.</span>
</pre></div>
</div>
<p>Notice the <tt class="docutils literal"><span class="pre">complete</span></tt> signal in <tt class="docutils literal"><span class="pre">go()</span></tt>?  No receivers have
connected to <tt class="docutils literal"><span class="pre">complete</span></tt> yet, and that&#8217;s a-ok.  Calling
<a class="reference internal" href="api.html#blinker.base.Signal.send" title="blinker.base.Signal.send"><tt class="xref py py-meth docutils literal"><span class="pre">send()</span></tt></a> on a signal with no receivers will result in no
notifications being sent, and these no-op sends are optimized to be as
inexpensive as possible.</p>
</div>
<div class="section" id="subscribing-to-specific-senders">
<h2>Subscribing to Specific Senders<a class="headerlink" href="#subscribing-to-specific-senders" title="Permalink to this headline"></a></h2>
<p>The default connection to a signal invokes the receiver function when
any sender emits it.  The <a class="reference internal" href="api.html#blinker.base.Signal.connect" title="blinker.base.Signal.connect"><tt class="xref py py-meth docutils literal"><span class="pre">Signal.connect()</span></tt></a> function accepts an
optional argument to restrict the subscription to one specific sending
object:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="k">def</span> <span class="nf">b_subscriber</span><span class="p">(</span><span class="n">sender</span><span class="p">):</span>
<span class="gp">... </span>    <span class="k">print</span><span class="p">(</span><span class="s">&quot;Caught signal from processor_b.&quot;</span><span class="p">)</span>
<span class="gp">... </span>    <span class="k">assert</span> <span class="n">sender</span><span class="o">.</span><span class="n">name</span> <span class="o">==</span> <span class="s">&#39;b&#39;</span>
<span class="gp">...</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">processor_b</span> <span class="o">=</span> <span class="n">Processor</span><span class="p">(</span><span class="s">&#39;b&#39;</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">ready</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="n">b_subscriber</span><span class="p">,</span> <span class="n">sender</span><span class="o">=</span><span class="n">processor_b</span><span class="p">)</span>
<span class="go">&lt;function b_subscriber at 0x...&gt;</span>
</pre></div>
</div>
<p>This function has been subscribed to <tt class="docutils literal"><span class="pre">ready</span></tt> but only when sent by
<tt class="docutils literal"><span class="pre">processor_b</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">processor_a</span><span class="o">.</span><span class="n">go</span><span class="p">()</span>
<span class="go">Got a signal sent by &lt;Processor a&gt;</span>
<span class="go">Processing.</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">processor_b</span><span class="o">.</span><span class="n">go</span><span class="p">()</span>
<span class="go">Got a signal sent by &lt;Processor b&gt;</span>
<span class="go">Caught signal from processor_b.</span>
<span class="go">Processing.</span>
</pre></div>
</div>
</div>
<div class="section" id="sending-and-receiving-data-through-signals">
<h2>Sending and Receiving Data Through Signals<a class="headerlink" href="#sending-and-receiving-data-through-signals" title="Permalink to this headline"></a></h2>
<p>Additional keyword arguments can be passed to <a class="reference internal" href="api.html#blinker.base.Signal.send" title="blinker.base.Signal.send"><tt class="xref py py-meth docutils literal"><span class="pre">send()</span></tt></a>.
These will in turn be passed to the connected functions:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">send_data</span> <span class="o">=</span> <span class="n">signal</span><span class="p">(</span><span class="s">&#39;send-data&#39;</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="nd">@send_data.connect</span>
<span class="gp">... </span><span class="k">def</span> <span class="nf">receive_data</span><span class="p">(</span><span class="n">sender</span><span class="p">,</span> <span class="o">**</span><span class="n">kw</span><span class="p">):</span>
<span class="gp">... </span>    <span class="k">print</span><span class="p">(</span><span class="s">&quot;Caught signal from </span><span class="si">%r</span><span class="s">, data </span><span class="si">%r</span><span class="s">&quot;</span> <span class="o">%</span> <span class="p">(</span><span class="n">sender</span><span class="p">,</span> <span class="n">kw</span><span class="p">))</span>
<span class="gp">... </span>    <span class="k">return</span> <span class="s">&#39;received!&#39;</span>
<span class="gp">...</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">result</span> <span class="o">=</span> <span class="n">send_data</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="s">&#39;anonymous&#39;</span><span class="p">,</span> <span class="n">abc</span><span class="o">=</span><span class="mi">123</span><span class="p">)</span>
<span class="go">Caught signal from &#39;anonymous&#39;, data {&#39;abc&#39;: 123}</span>
</pre></div>
</div>
<p>The return value of <a class="reference internal" href="api.html#blinker.base.Signal.send" title="blinker.base.Signal.send"><tt class="xref py py-meth docutils literal"><span class="pre">send()</span></tt></a> collects the return values of
each connected function as a list of (<tt class="docutils literal"><span class="pre">receiver</span> <span class="pre">function</span></tt>, <tt class="docutils literal"><span class="pre">return</span>
<span class="pre">value</span></tt>) pairs:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">result</span>
<span class="go">[(&lt;function receive_data at 0x...&gt;, &#39;received!&#39;)]</span>
</pre></div>
</div>
</div>
<div class="section" id="anonymous-signals">
<h2>Anonymous Signals<a class="headerlink" href="#anonymous-signals" title="Permalink to this headline"></a></h2>
<p>Signals need not be named.  The <a class="reference internal" href="api.html#blinker.base.Signal" title="blinker.base.Signal"><tt class="xref py py-class docutils literal"><span class="pre">Signal</span></tt></a> constructor creates a
unique signal each time it is invoked.  For example, an alternative
implementation of the Processor from above might provide the
processing signals as class attributes:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">blinker</span> <span class="kn">import</span> <span class="n">Signal</span>
<span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">AltProcessor</span><span class="p">:</span>
<span class="gp">... </span>   <span class="n">on_ready</span> <span class="o">=</span> <span class="n">Signal</span><span class="p">()</span>
<span class="gp">... </span>   <span class="n">on_complete</span> <span class="o">=</span> <span class="n">Signal</span><span class="p">()</span>
<span class="gp">...</span>
<span class="gp">... </span>   <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>
<span class="gp">... </span>       <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
<span class="gp">...</span>
<span class="gp">... </span>   <span class="k">def</span> <span class="nf">go</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span>       <span class="bp">self</span><span class="o">.</span><span class="n">on_ready</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span>
<span class="gp">... </span>       <span class="k">print</span><span class="p">(</span><span class="s">&quot;Alternate processing.&quot;</span><span class="p">)</span>
<span class="gp">... </span>       <span class="bp">self</span><span class="o">.</span><span class="n">on_complete</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">... </span>   <span class="k">def</span> <span class="nf">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span>       <span class="k">return</span> <span class="s">&#39;&lt;AltProcessor </span><span class="si">%s</span><span class="s">&gt;&#39;</span> <span class="o">%</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span>
<span class="gp">...</span>
</pre></div>
</div>
</div>
<div class="section" id="connect-as-a-decorator">
<h2><tt class="docutils literal"><span class="pre">connect</span></tt> as a Decorator<a class="headerlink" href="#connect-as-a-decorator" title="Permalink to this headline"></a></h2>
<p>You may have noticed the return value of <a class="reference internal" href="api.html#blinker.base.Signal.connect" title="blinker.base.Signal.connect"><tt class="xref py py-meth docutils literal"><span class="pre">connect()</span></tt></a> in
the console output in the sections above.  This allows <tt class="docutils literal"><span class="pre">connect</span></tt> to
be used as a decorator on functions:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">apc</span> <span class="o">=</span> <span class="n">AltProcessor</span><span class="p">(</span><span class="s">&#39;c&#39;</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="nd">@apc.on_complete.connect</span>
<span class="gp">... </span><span class="k">def</span> <span class="nf">completed</span><span class="p">(</span><span class="n">sender</span><span class="p">):</span>
<span class="gp">... </span>    <span class="k">print</span> <span class="s">&quot;AltProcessor </span><span class="si">%s</span><span class="s"> completed!&quot;</span> <span class="o">%</span> <span class="n">sender</span><span class="o">.</span><span class="n">name</span>
<span class="gp">...</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">apc</span><span class="o">.</span><span class="n">go</span><span class="p">()</span>
<span class="go">Alternate processing.</span>
<span class="go">AltProcessor c completed!</span>
</pre></div>
</div>
<p>While convenient, this form unfortunately does not allow the
<tt class="docutils literal"><span class="pre">sender</span></tt> or <tt class="docutils literal"><span class="pre">weak</span></tt> arguments to be customized for the connected
function.  For this, <a class="reference internal" href="api.html#blinker.base.Signal.connect_via" title="blinker.base.Signal.connect_via"><tt class="xref py py-meth docutils literal"><span class="pre">connect_via()</span></tt></a> can be used:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">dice_roll</span> <span class="o">=</span> <span class="n">signal</span><span class="p">(</span><span class="s">&#39;dice_roll&#39;</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="nd">@dice_roll.connect_via</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="gp">... </span><span class="nd">@dice_roll.connect_via</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>
<span class="gp">... </span><span class="nd">@dice_roll.connect_via</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="gp">... </span><span class="k">def</span> <span class="nf">odd_subscriber</span><span class="p">(</span><span class="n">sender</span><span class="p">):</span>
<span class="gp">... </span>    <span class="k">print</span><span class="p">(</span><span class="s">&quot;Observed dice roll </span><span class="si">%r</span><span class="s">.&quot;</span> <span class="o">%</span> <span class="n">sender</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">result</span> <span class="o">=</span> <span class="n">dice_roll</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>
<span class="go">Observed dice roll 3.</span>
</pre></div>
</div>
</div>
<div class="section" id="optimizing-signal-sending">
<h2>Optimizing Signal Sending<a class="headerlink" href="#optimizing-signal-sending" title="Permalink to this headline"></a></h2>
<p>Signals are optimized to send very quickly, whether receivers are
connected or not.  If the data to be sent down a signal is very
expensive, it can be more efficient to check to see if any receivers
are connected first by testing the <a class="reference internal" href="api.html#blinker.base.Signal.receivers" title="blinker.base.Signal.receivers"><tt class="xref py py-attr docutils literal"><span class="pre">receivers</span></tt></a> property:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="nb">bool</span><span class="p">(</span><span class="n">signal</span><span class="p">(</span><span class="s">&#39;ready&#39;</span><span class="p">)</span><span class="o">.</span><span class="n">receivers</span><span class="p">)</span>
<span class="go">True</span>
<span class="gp">&gt;&gt;&gt; </span><span class="nb">bool</span><span class="p">(</span><span class="n">signal</span><span class="p">(</span><span class="s">&#39;complete&#39;</span><span class="p">)</span><span class="o">.</span><span class="n">receivers</span><span class="p">)</span>
<span class="go">False</span>
<span class="gp">&gt;&gt;&gt; </span><span class="nb">bool</span><span class="p">(</span><span class="n">AltProcessor</span><span class="o">.</span><span class="n">on_complete</span><span class="o">.</span><span class="n">receivers</span><span class="p">)</span>
<span class="go">True</span>
</pre></div>
</div>
<p>Checking for a receiver listening for a particular sender is also
possible:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">signal</span><span class="p">(</span><span class="s">&#39;ready&#39;</span><span class="p">)</span><span class="o">.</span><span class="n">has_receivers_for</span><span class="p">(</span><span class="n">processor_a</span><span class="p">)</span>
<span class="go">True</span>
</pre></div>
</div>
</div>
<div class="section" id="documenting-signals">
<h2>Documenting Signals<a class="headerlink" href="#documenting-signals" title="Permalink to this headline"></a></h2>
<p>Both named and anonymous signals can be passed a <tt class="docutils literal"><span class="pre">doc</span></tt> argument at
construction to set the pydoc help text for the signal.  This
documentation will be picked up by most documentation generators (such
as sphinx) and is nice for documenting any additional data parameters
that will be sent down with the signal.</p>
<p>See the documentation of the <a class="reference internal" href="api.html#blinker.base.receiver_connected" title="blinker.base.receiver_connected"><tt class="xref py py-obj docutils literal"><span class="pre">receiver_connected</span></tt></a> built-in signal
for an example.</p>
</div>
<div class="section" id="more">
<h2>More<a class="headerlink" href="#more" title="Permalink to this headline"></a></h2>
<p>Disconnecting receivers from signals, introspection of connected
receivers, private namespaces for named signals and more are discussed
in the <a class="reference internal" href="api.html#api"><em>API Documentation</em></a>.</p>
</div>
</div>


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  <h3><a href="index.html">Table Of Contents</a></h3>
  <ul>
<li><a class="reference internal" href="#">Signals</a><ul>
<li><a class="reference internal" href="#decoupling-with-named-signals">Decoupling With Named Signals</a></li>
<li><a class="reference internal" href="#subscribing-to-signals">Subscribing to Signals</a></li>
<li><a class="reference internal" href="#emitting-signals">Emitting Signals</a></li>
<li><a class="reference internal" href="#subscribing-to-specific-senders">Subscribing to Specific Senders</a></li>
<li><a class="reference internal" href="#sending-and-receiving-data-through-signals">Sending and Receiving Data Through Signals</a></li>
<li><a class="reference internal" href="#anonymous-signals">Anonymous Signals</a></li>
<li><a class="reference internal" href="#connect-as-a-decorator"><tt class="docutils literal"><span class="pre">connect</span></tt> as a Decorator</a></li>
<li><a class="reference internal" href="#optimizing-signal-sending">Optimizing Signal Sending</a></li>
<li><a class="reference internal" href="#documenting-signals">Documenting Signals</a></li>
<li><a class="reference internal" href="#more">More</a></li>
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