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<div class="section" id="module-eventlet.event">
<h1><tt class="xref docutils literal"><span class="pre">event</span></tt> – Cross-greenthread primitive<a class="headerlink" href="#module-eventlet.event" title="Permalink to this headline">¶</a></h1>
<dl class="class">
<dt id="eventlet.event.Event">
<em class="property">class </em><tt class="descclassname">eventlet.event.</tt><tt class="descname">Event</tt><a class="headerlink" href="#eventlet.event.Event" title="Permalink to this definition">¶</a></dt>
<dd><p>An abstraction where an arbitrary number of coroutines
can wait for one event from another.</p>
<p>Events are similar to a Queue that can only hold one item, but differ
in two important ways:</p>
<ol class="arabic simple">
<li>calling <a title="eventlet.event.Event.send" class="reference internal" href="#eventlet.event.Event.send"><tt class="xref docutils literal"><span class="pre">send()</span></tt></a> never unschedules the current greenthread</li>
<li><a title="eventlet.event.Event.send" class="reference internal" href="#eventlet.event.Event.send"><tt class="xref docutils literal"><span class="pre">send()</span></tt></a> can only be called once; create a new event to send again.</li>
</ol>
<p>They are good for communicating results between coroutines, and
are the basis for how
<a title="eventlet.greenthread.GreenThread.wait" class="reference external" href="greenthread.html#eventlet.greenthread.GreenThread.wait"><tt class="xref docutils literal"><span class="pre">GreenThread.wait()</span></tt></a>
is implemented.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">eventlet</span> <span class="kn">import</span> <span class="n">event</span>
<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">eventlet</span>
<span class="gp">>>> </span><span class="n">evt</span> <span class="o">=</span> <span class="n">event</span><span class="o">.</span><span class="n">Event</span><span class="p">()</span>
<span class="gp">>>> </span><span class="k">def</span> <span class="nf">baz</span><span class="p">(</span><span class="n">b</span><span class="p">):</span>
<span class="gp">... </span> <span class="n">evt</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="n">b</span> <span class="o">+</span> <span class="mi">1</span><span class="p">)</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">_</span> <span class="o">=</span> <span class="n">eventlet</span><span class="o">.</span><span class="n">spawn_n</span><span class="p">(</span><span class="n">baz</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
<span class="go">4</span>
</pre></div>
</div>
<dl class="method">
<dt id="eventlet.event.Event.ready">
<tt class="descname">ready</tt><big>(</big><big>)</big><a class="headerlink" href="#eventlet.event.Event.ready" title="Permalink to this definition">¶</a></dt>
<dd>Return true if the <a title="eventlet.event.Event.wait" class="reference internal" href="#eventlet.event.Event.wait"><tt class="xref docutils literal"><span class="pre">wait()</span></tt></a> call will return immediately.
Used to avoid waiting for things that might take a while to time out.
For example, you can put a bunch of events into a list, and then visit
them all repeatedly, calling <a title="eventlet.event.Event.ready" class="reference internal" href="#eventlet.event.Event.ready"><tt class="xref docutils literal"><span class="pre">ready()</span></tt></a> until one returns <tt class="xref docutils literal"><span class="pre">True</span></tt>,
and then you can <a title="eventlet.event.Event.wait" class="reference internal" href="#eventlet.event.Event.wait"><tt class="xref docutils literal"><span class="pre">wait()</span></tt></a> on that one.</dd></dl>
<dl class="method">
<dt id="eventlet.event.Event.send">
<tt class="descname">send</tt><big>(</big><em>result=None</em>, <em>exc=None</em><big>)</big><a class="headerlink" href="#eventlet.event.Event.send" title="Permalink to this definition">¶</a></dt>
<dd><p>Makes arrangements for the waiters to be woken with the
result and then returns immediately to the parent.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">eventlet</span> <span class="kn">import</span> <span class="n">event</span>
<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">eventlet</span>
<span class="gp">>>> </span><span class="n">evt</span> <span class="o">=</span> <span class="n">event</span><span class="o">.</span><span class="n">Event</span><span class="p">()</span>
<span class="gp">>>> </span><span class="k">def</span> <span class="nf">waiter</span><span class="p">():</span>
<span class="gp">... </span> <span class="k">print</span> <span class="s">'about to wait'</span>
<span class="gp">... </span> <span class="n">result</span> <span class="o">=</span> <span class="n">evt</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
<span class="gp">... </span> <span class="k">print</span> <span class="s">'waited for'</span><span class="p">,</span> <span class="n">result</span>
<span class="gp">>>> </span><span class="n">_</span> <span class="o">=</span> <span class="n">eventlet</span><span class="o">.</span><span class="n">spawn</span><span class="p">(</span><span class="n">waiter</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">eventlet</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="go">about to wait</span>
<span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="s">'a'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">eventlet</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="go">waited for a</span>
</pre></div>
</div>
<p>It is an error to call <a title="eventlet.event.Event.send" class="reference internal" href="#eventlet.event.Event.send"><tt class="xref docutils literal"><span class="pre">send()</span></tt></a> multiple times on the same event.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="s">'whoops'</span><span class="p">)</span>
<span class="gt">Traceback (most recent call last):</span>
<span class="c">...</span>
<span class="nc">AssertionError</span>: <span class="n-Identifier">Trying to re-send() an already-triggered event.</span>
</pre></div>
</div>
<p>Use <tt class="xref docutils literal"><span class="pre">reset()</span></tt> between <a title="eventlet.event.Event.send" class="reference internal" href="#eventlet.event.Event.send"><tt class="xref docutils literal"><span class="pre">send()</span></tt></a> s to reuse an event object.</p>
</dd></dl>
<dl class="method">
<dt id="eventlet.event.Event.send_exception">
<tt class="descname">send_exception</tt><big>(</big><em>*args</em><big>)</big><a class="headerlink" href="#eventlet.event.Event.send_exception" title="Permalink to this definition">¶</a></dt>
<dd><p>Same as <a title="eventlet.event.Event.send" class="reference internal" href="#eventlet.event.Event.send"><tt class="xref docutils literal"><span class="pre">send()</span></tt></a>, but sends an exception to waiters.</p>
<p>The arguments to send_exception are the same as the arguments
to <tt class="docutils literal"><span class="pre">raise</span></tt>. If a single exception object is passed in, it
will be re-raised when <a title="eventlet.event.Event.wait" class="reference internal" href="#eventlet.event.Event.wait"><tt class="xref docutils literal"><span class="pre">wait()</span></tt></a> is called, generating a
new stacktrace.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">eventlet</span> <span class="kn">import</span> <span class="n">event</span>
<span class="gp">>>> </span><span class="n">evt</span> <span class="o">=</span> <span class="n">event</span><span class="o">.</span><span class="n">Event</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">send_exception</span><span class="p">(</span><span class="ne">RuntimeError</span><span class="p">())</span>
<span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
<span class="gt">Traceback (most recent call last):</span>
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n-Identifier"><module></span>
File <span class="nb">"eventlet/event.py"</span>, line <span class="m">120</span>, in <span class="n-Identifier">wait</span>
<span class="n">current</span><span class="o">.</span><span class="n">throw</span><span class="p">(</span><span class="o">*</span><span class="bp">self</span><span class="o">.</span><span class="n">_exc</span><span class="p">)</span>
<span class="nc">RuntimeError</span>
</pre></div>
</div>
<p>If it’s important to preserve the entire original stack trace,
you must pass in the entire <tt class="xref docutils literal"><span class="pre">sys.exc_info()</span></tt> tuple.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">sys</span>
<span class="gp">>>> </span><span class="n">evt</span> <span class="o">=</span> <span class="n">event</span><span class="o">.</span><span class="n">Event</span><span class="p">()</span>
<span class="gp">>>> </span><span class="k">try</span><span class="p">:</span>
<span class="gp">... </span> <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">()</span>
<span class="gp">... </span><span class="k">except</span> <span class="ne">RuntimeError</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">evt</span><span class="o">.</span><span class="n">send_exception</span><span class="p">(</span><span class="o">*</span><span class="n">sys</span><span class="o">.</span><span class="n">exc_info</span><span class="p">())</span>
<span class="gp">... </span>
<span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
<span class="gt">Traceback (most recent call last):</span>
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n-Identifier"><module></span>
File <span class="nb">"eventlet/event.py"</span>, line <span class="m">120</span>, in <span class="n-Identifier">wait</span>
<span class="n">current</span><span class="o">.</span><span class="n">throw</span><span class="p">(</span><span class="o">*</span><span class="bp">self</span><span class="o">.</span><span class="n">_exc</span><span class="p">)</span>
File <span class="nb">"<stdin>"</span>, line <span class="m">2</span>, in <span class="n-Identifier"><module></span>
<span class="nc">RuntimeError</span>
</pre></div>
</div>
<p>Note that doing so stores a traceback object directly on the
Event object, which may cause reference cycles. See the
<tt class="xref docutils literal"><span class="pre">sys.exc_info()</span></tt> documentation.</p>
</dd></dl>
<dl class="method">
<dt id="eventlet.event.Event.wait">
<tt class="descname">wait</tt><big>(</big><big>)</big><a class="headerlink" href="#eventlet.event.Event.wait" title="Permalink to this definition">¶</a></dt>
<dd><p>Wait until another coroutine calls <a title="eventlet.event.Event.send" class="reference internal" href="#eventlet.event.Event.send"><tt class="xref docutils literal"><span class="pre">send()</span></tt></a>.
Returns the value the other coroutine passed to
<a title="eventlet.event.Event.send" class="reference internal" href="#eventlet.event.Event.send"><tt class="xref docutils literal"><span class="pre">send()</span></tt></a>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">eventlet</span> <span class="kn">import</span> <span class="n">event</span>
<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">eventlet</span>
<span class="gp">>>> </span><span class="n">evt</span> <span class="o">=</span> <span class="n">event</span><span class="o">.</span><span class="n">Event</span><span class="p">()</span>
<span class="gp">>>> </span><span class="k">def</span> <span class="nf">wait_on</span><span class="p">():</span>
<span class="gp">... </span> <span class="n">retval</span> <span class="o">=</span> <span class="n">evt</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
<span class="gp">... </span> <span class="k">print</span> <span class="s">"waited for"</span><span class="p">,</span> <span class="n">retval</span>
<span class="gp">>>> </span><span class="n">_</span> <span class="o">=</span> <span class="n">eventlet</span><span class="o">.</span><span class="n">spawn</span><span class="p">(</span><span class="n">wait_on</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="s">'result'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">eventlet</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="go">waited for result</span>
</pre></div>
</div>
<p>Returns immediately if the event has already
occured.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
<span class="go">'result'</span>
</pre></div>
</div>
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