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<div class="section" id="module-asynchat">
<span id="asynchat-asynchronous-socket-command-response-handler"></span><h1>17.7. <a class="reference internal" href="#module-asynchat" title="asynchat: Support for asynchronous command/response protocols."><tt class="xref py py-mod docutils literal"><span class="pre">asynchat</span></tt></a> — Asynchronous socket command/response handler<a class="headerlink" href="#module-asynchat" title="Permalink to this headline">¶</a></h1>
<p><strong>Source code:</strong> <a class="reference external" href="http://hg.python.org/cpython/file/2.7/Lib/asynchat.py">Lib/asynchat.py</a></p>
<hr class="docutils" />
<p>This module builds on the <a class="reference internal" href="asyncore.html#module-asyncore" title="asyncore: A base class for developing asynchronous socket handling services."><tt class="xref py py-mod docutils literal"><span class="pre">asyncore</span></tt></a> infrastructure, simplifying
asynchronous clients and servers and making it easier to handle protocols
whose elements are terminated by arbitrary strings, or are of variable length.
<a class="reference internal" href="#module-asynchat" title="asynchat: Support for asynchronous command/response protocols."><tt class="xref py py-mod docutils literal"><span class="pre">asynchat</span></tt></a> defines the abstract class <a class="reference internal" href="#asynchat.async_chat" title="asynchat.async_chat"><tt class="xref py py-class docutils literal"><span class="pre">async_chat</span></tt></a> that you
subclass, providing implementations of the <tt class="xref py py-meth docutils literal"><span class="pre">collect_incoming_data()</span></tt> and
<tt class="xref py py-meth docutils literal"><span class="pre">found_terminator()</span></tt> methods. It uses the same asynchronous loop as
<a class="reference internal" href="asyncore.html#module-asyncore" title="asyncore: A base class for developing asynchronous socket handling services."><tt class="xref py py-mod docutils literal"><span class="pre">asyncore</span></tt></a>, and the two types of channel, <a class="reference internal" href="asyncore.html#asyncore.dispatcher" title="asyncore.dispatcher"><tt class="xref py py-class docutils literal"><span class="pre">asyncore.dispatcher</span></tt></a>
and <a class="reference internal" href="#asynchat.async_chat" title="asynchat.async_chat"><tt class="xref py py-class docutils literal"><span class="pre">asynchat.async_chat</span></tt></a>, can freely be mixed in the channel map.
Typically an <a class="reference internal" href="asyncore.html#asyncore.dispatcher" title="asyncore.dispatcher"><tt class="xref py py-class docutils literal"><span class="pre">asyncore.dispatcher</span></tt></a> server channel generates new
<a class="reference internal" href="#asynchat.async_chat" title="asynchat.async_chat"><tt class="xref py py-class docutils literal"><span class="pre">asynchat.async_chat</span></tt></a> channel objects as it receives incoming
connection requests.</p>
<dl class="class">
<dt id="asynchat.async_chat">
<em class="property">class </em><tt class="descclassname">asynchat.</tt><tt class="descname">async_chat</tt><a class="headerlink" href="#asynchat.async_chat" title="Permalink to this definition">¶</a></dt>
<dd><p>This class is an abstract subclass of <a class="reference internal" href="asyncore.html#asyncore.dispatcher" title="asyncore.dispatcher"><tt class="xref py py-class docutils literal"><span class="pre">asyncore.dispatcher</span></tt></a>. To make
practical use of the code you must subclass <a class="reference internal" href="#asynchat.async_chat" title="asynchat.async_chat"><tt class="xref py py-class docutils literal"><span class="pre">async_chat</span></tt></a>, providing
meaningful <a class="reference internal" href="#asynchat.async_chat.collect_incoming_data" title="asynchat.async_chat.collect_incoming_data"><tt class="xref py py-meth docutils literal"><span class="pre">collect_incoming_data()</span></tt></a> and <a class="reference internal" href="#asynchat.async_chat.found_terminator" title="asynchat.async_chat.found_terminator"><tt class="xref py py-meth docutils literal"><span class="pre">found_terminator()</span></tt></a>
methods.
The <a class="reference internal" href="asyncore.html#asyncore.dispatcher" title="asyncore.dispatcher"><tt class="xref py py-class docutils literal"><span class="pre">asyncore.dispatcher</span></tt></a> methods can be used, although not all make
sense in a message/response context.</p>
<p>Like <a class="reference internal" href="asyncore.html#asyncore.dispatcher" title="asyncore.dispatcher"><tt class="xref py py-class docutils literal"><span class="pre">asyncore.dispatcher</span></tt></a>, <a class="reference internal" href="#asynchat.async_chat" title="asynchat.async_chat"><tt class="xref py py-class docutils literal"><span class="pre">async_chat</span></tt></a> defines a set of
events that are generated by an analysis of socket conditions after a
<tt class="xref c c-func docutils literal"><span class="pre">select()</span></tt> call. Once the polling loop has been started the
<a class="reference internal" href="#asynchat.async_chat" title="asynchat.async_chat"><tt class="xref py py-class docutils literal"><span class="pre">async_chat</span></tt></a> object’s methods are called by the event-processing
framework with no action on the part of the programmer.</p>
<p>Two class attributes can be modified, to improve performance, or possibly
even to conserve memory.</p>
<dl class="data">
<dt id="asynchat.async_chat.ac_in_buffer_size">
<tt class="descname">ac_in_buffer_size</tt><a class="headerlink" href="#asynchat.async_chat.ac_in_buffer_size" title="Permalink to this definition">¶</a></dt>
<dd><p>The asynchronous input buffer size (default <tt class="docutils literal"><span class="pre">4096</span></tt>).</p>
</dd></dl>
<dl class="data">
<dt id="asynchat.async_chat.ac_out_buffer_size">
<tt class="descname">ac_out_buffer_size</tt><a class="headerlink" href="#asynchat.async_chat.ac_out_buffer_size" title="Permalink to this definition">¶</a></dt>
<dd><p>The asynchronous output buffer size (default <tt class="docutils literal"><span class="pre">4096</span></tt>).</p>
</dd></dl>
<p>Unlike <a class="reference internal" href="asyncore.html#asyncore.dispatcher" title="asyncore.dispatcher"><tt class="xref py py-class docutils literal"><span class="pre">asyncore.dispatcher</span></tt></a>, <a class="reference internal" href="#asynchat.async_chat" title="asynchat.async_chat"><tt class="xref py py-class docutils literal"><span class="pre">async_chat</span></tt></a> allows you to
define a first-in-first-out queue (fifo) of <em>producers</em>. A producer need
have only one method, <tt class="xref py py-meth docutils literal"><span class="pre">more()</span></tt>, which should return data to be
transmitted on the channel.
The producer indicates exhaustion (<em>i.e.</em> that it contains no more data) by
having its <tt class="xref py py-meth docutils literal"><span class="pre">more()</span></tt> method return the empty string. At this point the
<a class="reference internal" href="#asynchat.async_chat" title="asynchat.async_chat"><tt class="xref py py-class docutils literal"><span class="pre">async_chat</span></tt></a> object removes the producer from the fifo and starts
using the next producer, if any. When the producer fifo is empty the
<tt class="xref py py-meth docutils literal"><span class="pre">handle_write()</span></tt> method does nothing. You use the channel object’s
<a class="reference internal" href="#asynchat.async_chat.set_terminator" title="asynchat.async_chat.set_terminator"><tt class="xref py py-meth docutils literal"><span class="pre">set_terminator()</span></tt></a> method to describe how to recognize the end of, or
an important breakpoint in, an incoming transmission from the remote
endpoint.</p>
<p>To build a functioning <a class="reference internal" href="#asynchat.async_chat" title="asynchat.async_chat"><tt class="xref py py-class docutils literal"><span class="pre">async_chat</span></tt></a> subclass your input methods
<a class="reference internal" href="#asynchat.async_chat.collect_incoming_data" title="asynchat.async_chat.collect_incoming_data"><tt class="xref py py-meth docutils literal"><span class="pre">collect_incoming_data()</span></tt></a> and <a class="reference internal" href="#asynchat.async_chat.found_terminator" title="asynchat.async_chat.found_terminator"><tt class="xref py py-meth docutils literal"><span class="pre">found_terminator()</span></tt></a> must handle the
data that the channel receives asynchronously. The methods are described
below.</p>
</dd></dl>
<dl class="method">
<dt id="asynchat.async_chat.close_when_done">
<tt class="descclassname">async_chat.</tt><tt class="descname">close_when_done</tt><big>(</big><big>)</big><a class="headerlink" href="#asynchat.async_chat.close_when_done" title="Permalink to this definition">¶</a></dt>
<dd><p>Pushes a <tt class="docutils literal"><span class="pre">None</span></tt> on to the producer fifo. When this producer is popped off
the fifo it causes the channel to be closed.</p>
</dd></dl>
<dl class="method">
<dt id="asynchat.async_chat.collect_incoming_data">
<tt class="descclassname">async_chat.</tt><tt class="descname">collect_incoming_data</tt><big>(</big><em>data</em><big>)</big><a class="headerlink" href="#asynchat.async_chat.collect_incoming_data" title="Permalink to this definition">¶</a></dt>
<dd><p>Called with <em>data</em> holding an arbitrary amount of received data. The
default method, which must be overridden, raises a
<a class="reference internal" href="exceptions.html#exceptions.NotImplementedError" title="exceptions.NotImplementedError"><tt class="xref py py-exc docutils literal"><span class="pre">NotImplementedError</span></tt></a> exception.</p>
</dd></dl>
<dl class="method">
<dt id="asynchat.async_chat.discard_buffers">
<tt class="descclassname">async_chat.</tt><tt class="descname">discard_buffers</tt><big>(</big><big>)</big><a class="headerlink" href="#asynchat.async_chat.discard_buffers" title="Permalink to this definition">¶</a></dt>
<dd><p>In emergencies this method will discard any data held in the input and/or
output buffers and the producer fifo.</p>
</dd></dl>
<dl class="method">
<dt id="asynchat.async_chat.found_terminator">
<tt class="descclassname">async_chat.</tt><tt class="descname">found_terminator</tt><big>(</big><big>)</big><a class="headerlink" href="#asynchat.async_chat.found_terminator" title="Permalink to this definition">¶</a></dt>
<dd><p>Called when the incoming data stream matches the termination condition set
by <a class="reference internal" href="#asynchat.async_chat.set_terminator" title="asynchat.async_chat.set_terminator"><tt class="xref py py-meth docutils literal"><span class="pre">set_terminator()</span></tt></a>. The default method, which must be overridden,
raises a <a class="reference internal" href="exceptions.html#exceptions.NotImplementedError" title="exceptions.NotImplementedError"><tt class="xref py py-exc docutils literal"><span class="pre">NotImplementedError</span></tt></a> exception. The buffered input data
should be available via an instance attribute.</p>
</dd></dl>
<dl class="method">
<dt id="asynchat.async_chat.get_terminator">
<tt class="descclassname">async_chat.</tt><tt class="descname">get_terminator</tt><big>(</big><big>)</big><a class="headerlink" href="#asynchat.async_chat.get_terminator" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns the current terminator for the channel.</p>
</dd></dl>
<dl class="method">
<dt id="asynchat.async_chat.push">
<tt class="descclassname">async_chat.</tt><tt class="descname">push</tt><big>(</big><em>data</em><big>)</big><a class="headerlink" href="#asynchat.async_chat.push" title="Permalink to this definition">¶</a></dt>
<dd><p>Pushes data on to the channel’s fifo to ensure its transmission.
This is all you need to do to have the channel write the data out to the
network, although it is possible to use your own producers in more complex
schemes to implement encryption and chunking, for example.</p>
</dd></dl>
<dl class="method">
<dt id="asynchat.async_chat.push_with_producer">
<tt class="descclassname">async_chat.</tt><tt class="descname">push_with_producer</tt><big>(</big><em>producer</em><big>)</big><a class="headerlink" href="#asynchat.async_chat.push_with_producer" title="Permalink to this definition">¶</a></dt>
<dd><p>Takes a producer object and adds it to the producer fifo associated with
the channel. When all currently-pushed producers have been exhausted the
channel will consume this producer’s data by calling its <tt class="xref py py-meth docutils literal"><span class="pre">more()</span></tt>
method and send the data to the remote endpoint.</p>
</dd></dl>
<dl class="method">
<dt id="asynchat.async_chat.set_terminator">
<tt class="descclassname">async_chat.</tt><tt class="descname">set_terminator</tt><big>(</big><em>term</em><big>)</big><a class="headerlink" href="#asynchat.async_chat.set_terminator" title="Permalink to this definition">¶</a></dt>
<dd><p>Sets the terminating condition to be recognized on the channel. <tt class="docutils literal"><span class="pre">term</span></tt>
may be any of three types of value, corresponding to three different ways
to handle incoming protocol data.</p>
<table border="1" class="docutils">
<colgroup>
<col width="20%" />
<col width="80%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">term</th>
<th class="head">Description</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><em>string</em></td>
<td>Will call <a class="reference internal" href="#asynchat.async_chat.found_terminator" title="asynchat.async_chat.found_terminator"><tt class="xref py py-meth docutils literal"><span class="pre">found_terminator()</span></tt></a> when the
string is found in the input stream</td>
</tr>
<tr class="row-odd"><td><em>integer</em></td>
<td>Will call <a class="reference internal" href="#asynchat.async_chat.found_terminator" title="asynchat.async_chat.found_terminator"><tt class="xref py py-meth docutils literal"><span class="pre">found_terminator()</span></tt></a> when the
indicated number of characters have been
received</td>
</tr>
<tr class="row-even"><td><tt class="docutils literal"><span class="pre">None</span></tt></td>
<td>The channel continues to collect data
forever</td>
</tr>
</tbody>
</table>
<p>Note that any data following the terminator will be available for reading
by the channel after <a class="reference internal" href="#asynchat.async_chat.found_terminator" title="asynchat.async_chat.found_terminator"><tt class="xref py py-meth docutils literal"><span class="pre">found_terminator()</span></tt></a> is called.</p>
</dd></dl>
<div class="section" id="asynchat-auxiliary-classes">
<h2>17.7.1. asynchat - Auxiliary Classes<a class="headerlink" href="#asynchat-auxiliary-classes" title="Permalink to this headline">¶</a></h2>
<dl class="class">
<dt id="asynchat.fifo">
<em class="property">class </em><tt class="descclassname">asynchat.</tt><tt class="descname">fifo</tt><big>(</big><span class="optional">[</span><em>list=None</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#asynchat.fifo" title="Permalink to this definition">¶</a></dt>
<dd><p>A <a class="reference internal" href="#asynchat.fifo" title="asynchat.fifo"><tt class="xref py py-class docutils literal"><span class="pre">fifo</span></tt></a> holding data which has been pushed by the application but
not yet popped for writing to the channel. A <a class="reference internal" href="#asynchat.fifo" title="asynchat.fifo"><tt class="xref py py-class docutils literal"><span class="pre">fifo</span></tt></a> is a list used
to hold data and/or producers until they are required. If the <em>list</em>
argument is provided then it should contain producers or data items to be
written to the channel.</p>
<dl class="method">
<dt id="asynchat.fifo.is_empty">
<tt class="descname">is_empty</tt><big>(</big><big>)</big><a class="headerlink" href="#asynchat.fifo.is_empty" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns <tt class="docutils literal"><span class="pre">True</span></tt> if and only if the fifo is empty.</p>
</dd></dl>
<dl class="method">
<dt id="asynchat.fifo.first">
<tt class="descname">first</tt><big>(</big><big>)</big><a class="headerlink" href="#asynchat.fifo.first" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns the least-recently <a class="reference internal" href="#asynchat.fifo.push" title="asynchat.fifo.push"><tt class="xref py py-meth docutils literal"><span class="pre">push()</span></tt></a>ed item from the fifo.</p>
</dd></dl>
<dl class="method">
<dt id="asynchat.fifo.push">
<tt class="descname">push</tt><big>(</big><em>data</em><big>)</big><a class="headerlink" href="#asynchat.fifo.push" title="Permalink to this definition">¶</a></dt>
<dd><p>Adds the given data (which may be a string or a producer object) to the
producer fifo.</p>
</dd></dl>
<dl class="method">
<dt id="asynchat.fifo.pop">
<tt class="descname">pop</tt><big>(</big><big>)</big><a class="headerlink" href="#asynchat.fifo.pop" title="Permalink to this definition">¶</a></dt>
<dd><p>If the fifo is not empty, returns <tt class="docutils literal"><span class="pre">True,</span> <span class="pre">first()</span></tt>, deleting the popped
item. Returns <tt class="docutils literal"><span class="pre">False,</span> <span class="pre">None</span></tt> for an empty fifo.</p>
</dd></dl>
</dd></dl>
</div>
<div class="section" id="asynchat-example">
<span id="id1"></span><h2>17.7.2. asynchat Example<a class="headerlink" href="#asynchat-example" title="Permalink to this headline">¶</a></h2>
<p>The following partial example shows how HTTP requests can be read with
<a class="reference internal" href="#asynchat.async_chat" title="asynchat.async_chat"><tt class="xref py py-class docutils literal"><span class="pre">async_chat</span></tt></a>. A web server might create an
<tt class="xref py py-class docutils literal"><span class="pre">http_request_handler</span></tt> object for each incoming client connection.
Notice that initially the channel terminator is set to match the blank line at
the end of the HTTP headers, and a flag indicates that the headers are being
read.</p>
<p>Once the headers have been read, if the request is of type POST (indicating
that further data are present in the input stream) then the
<tt class="docutils literal"><span class="pre">Content-Length:</span></tt> header is used to set a numeric terminator to read the
right amount of data from the channel.</p>
<p>The <tt class="xref py py-meth docutils literal"><span class="pre">handle_request()</span></tt> method is called once all relevant input has been
marshalled, after setting the channel terminator to <tt class="docutils literal"><span class="pre">None</span></tt> to ensure that
any extraneous data sent by the web client are ignored.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">http_request_handler</span><span class="p">(</span><span class="n">asynchat</span><span class="o">.</span><span class="n">async_chat</span><span class="p">):</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">sock</span><span class="p">,</span> <span class="n">addr</span><span class="p">,</span> <span class="n">sessions</span><span class="p">,</span> <span class="n">log</span><span class="p">):</span>
<span class="n">asynchat</span><span class="o">.</span><span class="n">async_chat</span><span class="o">.</span><span class="n">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">sock</span><span class="o">=</span><span class="n">sock</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">addr</span> <span class="o">=</span> <span class="n">addr</span>
<span class="bp">self</span><span class="o">.</span><span class="n">sessions</span> <span class="o">=</span> <span class="n">sessions</span>
<span class="bp">self</span><span class="o">.</span><span class="n">ibuffer</span> <span class="o">=</span> <span class="p">[]</span>
<span class="bp">self</span><span class="o">.</span><span class="n">obuffer</span> <span class="o">=</span> <span class="s">""</span>
<span class="bp">self</span><span class="o">.</span><span class="n">set_terminator</span><span class="p">(</span><span class="s">"</span><span class="se">\r\n\r\n</span><span class="s">"</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">reading_headers</span> <span class="o">=</span> <span class="bp">True</span>
<span class="bp">self</span><span class="o">.</span><span class="n">handling</span> <span class="o">=</span> <span class="bp">False</span>
<span class="bp">self</span><span class="o">.</span><span class="n">cgi_data</span> <span class="o">=</span> <span class="bp">None</span>
<span class="bp">self</span><span class="o">.</span><span class="n">log</span> <span class="o">=</span> <span class="n">log</span>
<span class="k">def</span> <span class="nf">collect_incoming_data</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">data</span><span class="p">):</span>
<span class="sd">"""Buffer the data"""</span>
<span class="bp">self</span><span class="o">.</span><span class="n">ibuffer</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">found_terminator</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">reading_headers</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">reading_headers</span> <span class="o">=</span> <span class="bp">False</span>
<span class="bp">self</span><span class="o">.</span><span class="n">parse_headers</span><span class="p">(</span><span class="s">""</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">ibuffer</span><span class="p">))</span>
<span class="bp">self</span><span class="o">.</span><span class="n">ibuffer</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">op</span><span class="o">.</span><span class="n">upper</span><span class="p">()</span> <span class="o">==</span> <span class="s">"POST"</span><span class="p">:</span>
<span class="n">clen</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">headers</span><span class="o">.</span><span class="n">getheader</span><span class="p">(</span><span class="s">"content-length"</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">set_terminator</span><span class="p">(</span><span class="nb">int</span><span class="p">(</span><span class="n">clen</span><span class="p">))</span>
<span class="k">else</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">handling</span> <span class="o">=</span> <span class="bp">True</span>
<span class="bp">self</span><span class="o">.</span><span class="n">set_terminator</span><span class="p">(</span><span class="bp">None</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">handle_request</span><span class="p">()</span>
<span class="k">elif</span> <span class="ow">not</span> <span class="bp">self</span><span class="o">.</span><span class="n">handling</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">set_terminator</span><span class="p">(</span><span class="bp">None</span><span class="p">)</span> <span class="c"># browsers sometimes over-send</span>
<span class="bp">self</span><span class="o">.</span><span class="n">cgi_data</span> <span class="o">=</span> <span class="n">parse</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">headers</span><span class="p">,</span> <span class="s">""</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">ibuffer</span><span class="p">))</span>
<span class="bp">self</span><span class="o">.</span><span class="n">handling</span> <span class="o">=</span> <span class="bp">True</span>
<span class="bp">self</span><span class="o">.</span><span class="n">ibuffer</span> <span class="o">=</span> <span class="p">[]</span>
<span class="bp">self</span><span class="o">.</span><span class="n">handle_request</span><span class="p">()</span>
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