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<ul>
<li><a class="reference internal" href="#">threading – Manage concurrent threads</a><ul>
<li><a class="reference internal" href="#thread-objects">Thread Objects</a></li>
<li><a class="reference internal" href="#determining-the-current-thread">Determining the Current Thread</a></li>
<li><a class="reference internal" href="#daemon-vs-non-daemon-threads">Daemon vs. Non-Daemon Threads</a></li>
<li><a class="reference internal" href="#enumerating-all-threads">Enumerating All Threads</a></li>
<li><a class="reference internal" href="#subclassing-thread">Subclassing Thread</a></li>
<li><a class="reference internal" href="#timer-threads">Timer Threads</a></li>
<li><a class="reference internal" href="#signaling-between-threads">Signaling Between Threads</a></li>
<li><a class="reference internal" href="#controlling-access-to-resources">Controlling Access to Resources</a><ul>
<li><a class="reference internal" href="#re-entrant-locks">Re-entrant Locks</a></li>
<li><a class="reference internal" href="#locks-as-context-managers">Locks as Context Managers</a></li>
</ul>
</li>
<li><a class="reference internal" href="#synchronizing-threads">Synchronizing Threads</a></li>
<li><a class="reference internal" href="#limiting-concurrent-access-to-resources">Limiting Concurrent Access to Resources</a></li>
<li><a class="reference internal" href="#thread-specific-data">Thread-specific Data</a></li>
</ul>
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<div class="section" id="module-threading">
<span id="threading-manage-concurrent-threads"></span><h1>threading – Manage concurrent threads<a class="headerlink" href="#module-threading" title="Permalink to this headline">¶</a></h1>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Purpose:</th><td class="field-body">Builds on the <tt class="xref py py-mod docutils literal"><span class="pre">thread</span></tt> module to more easily manage several threads of execution.</td>
</tr>
<tr class="field"><th class="field-name">Python Version:</th><td class="field-body">1.5.2 and later</td>
</tr>
</tbody>
</table>
<p>The <a class="reference internal" href="#module-threading" title="threading: Manage several concurrent threads of execution."><tt class="xref py py-mod docutils literal"><span class="pre">threading</span></tt></a> module builds on the low-level features of
<tt class="xref py py-mod docutils literal"><span class="pre">thread</span></tt> to make working with threads even easier and more
<em>pythonic</em>. Using threads allows a program to run multiple operations
concurrently in the same process space.</p>
<div class="section" id="thread-objects">
<h2>Thread Objects<a class="headerlink" href="#thread-objects" title="Permalink to this headline">¶</a></h2>
<p>The simplest way to use a <tt class="xref py py-class docutils literal"><span class="pre">Thread</span></tt> is to instantiate it with a
target function and call <tt class="xref py py-func docutils literal"><span class="pre">start()</span></tt> to let it begin working.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">threading</span>
<span class="k">def</span> <span class="nf">worker</span><span class="p">():</span>
<span class="sd">"""thread worker function"""</span>
<span class="k">print</span> <span class="s">'Worker'</span>
<span class="k">return</span>
<span class="n">threads</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</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">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">)</span>
<span class="n">threads</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">t</span><span class="p">)</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
</pre></div>
</div>
<p>The output is five lines with <tt class="docutils literal"><span class="pre">"Worker"</span></tt> on each:</p>
<div class="highlight-python"><pre>$ python threading_simple.py
Worker
Worker
Worker
Worker
Worker</pre>
</div>
<p>It useful to be able to spawn a thread and pass it arguments to tell
it what work to do. This example passes a number, which the thread
then prints.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">threading</span>
<span class="k">def</span> <span class="nf">worker</span><span class="p">(</span><span class="n">num</span><span class="p">):</span>
<span class="sd">"""thread worker function"""</span>
<span class="k">print</span> <span class="s">'Worker: </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">num</span>
<span class="k">return</span>
<span class="n">threads</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</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">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">i</span><span class="p">,))</span>
<span class="n">threads</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">t</span><span class="p">)</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
</pre></div>
</div>
<p>The integer argument is now included in the message printed by each
thread:</p>
<div class="highlight-python"><pre> $ python -u threading_simpleargs.py
Worker: 0
Worker: 1
Worker: 2
Worker: 3
Worker: 4</pre>
</div>
</div>
<div class="section" id="determining-the-current-thread">
<h2>Determining the Current Thread<a class="headerlink" href="#determining-the-current-thread" title="Permalink to this headline">¶</a></h2>
<p>Using arguments to identify or name the thread is cumbersome, and
unnecessary. Each <tt class="xref py py-class docutils literal"><span class="pre">Thread</span></tt> instance has a name with a default
value that can be changed as the thread is created. Naming threads is
useful in server processes with multiple service threads handling
different operations.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="k">def</span> <span class="nf">worker</span><span class="p">():</span>
<span class="k">print</span> <span class="n">threading</span><span class="o">.</span><span class="n">currentThread</span><span class="p">()</span><span class="o">.</span><span class="n">getName</span><span class="p">(),</span> <span class="s">'Starting'</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="k">print</span> <span class="n">threading</span><span class="o">.</span><span class="n">currentThread</span><span class="p">()</span><span class="o">.</span><span class="n">getName</span><span class="p">(),</span> <span class="s">'Exiting'</span>
<span class="k">def</span> <span class="nf">my_service</span><span class="p">():</span>
<span class="k">print</span> <span class="n">threading</span><span class="o">.</span><span class="n">currentThread</span><span class="p">()</span><span class="o">.</span><span class="n">getName</span><span class="p">(),</span> <span class="s">'Starting'</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>
<span class="k">print</span> <span class="n">threading</span><span class="o">.</span><span class="n">currentThread</span><span class="p">()</span><span class="o">.</span><span class="n">getName</span><span class="p">(),</span> <span class="s">'Exiting'</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'my_service'</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">my_service</span><span class="p">)</span>
<span class="n">w</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'worker'</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">)</span>
<span class="n">w2</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">)</span> <span class="c"># use default name</span>
<span class="n">w</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">w2</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
</pre></div>
</div>
<p>The debug output includes the name of the current thread on each
line. The lines with <tt class="docutils literal"><span class="pre">"Thread-1"</span></tt> in the thread name column
correspond to the unnamed thread <tt class="xref py py-data docutils literal"><span class="pre">w2</span></tt>.</p>
<div class="highlight-python"><pre>$ python -u threading_names.py
worker Starting
Thread-1 Starting
my_service Starting
workerThread-1 Exiting
Exiting
my_service Exiting</pre>
</div>
<p>Most programs do not use <strong class="command">print</strong> to debug. The
<a class="reference internal" href="../logging/index.html#module-logging" title="logging: Report status, error, and informational messages."><tt class="xref py py-mod docutils literal"><span class="pre">logging</span></tt></a> module supports embedding the thread name in every log
message using the formatter code <tt class="docutils literal"><span class="pre">%(threadName)s</span></tt>. Including thread
names in log messages makes it easier to trace those messages back to
their source.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'[</span><span class="si">%(levelname)s</span><span class="s">] (</span><span class="si">%(threadName)-10s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">def</span> <span class="nf">worker</span><span class="p">():</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Starting'</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Exiting'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">my_service</span><span class="p">():</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Starting'</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Exiting'</span><span class="p">)</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'my_service'</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">my_service</span><span class="p">)</span>
<span class="n">w</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'worker'</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">)</span>
<span class="n">w2</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">)</span> <span class="c"># use default name</span>
<span class="n">w</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">w2</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
</pre></div>
</div>
<p><a class="reference internal" href="../logging/index.html#module-logging" title="logging: Report status, error, and informational messages."><tt class="xref py py-mod docutils literal"><span class="pre">logging</span></tt></a> is also thread-safe, so messages from different threads
are kept distinct in the output.</p>
<div class="highlight-python"><pre>$ python threading_names_log.py
[DEBUG] (worker ) Starting
[DEBUG] (Thread-1 ) Starting
[DEBUG] (my_service) Starting
[DEBUG] (worker ) Exiting
[DEBUG] (Thread-1 ) Exiting
[DEBUG] (my_service) Exiting</pre>
</div>
</div>
<div class="section" id="daemon-vs-non-daemon-threads">
<h2>Daemon vs. Non-Daemon Threads<a class="headerlink" href="#daemon-vs-non-daemon-threads" title="Permalink to this headline">¶</a></h2>
<p>Up to this point, the example programs have implicitly waited to exit
until all threads have completed their work. Sometimes programs spawn
a thread as a <em>daemon</em> that runs without blocking the main program
from exiting. Using daemon threads is useful for services where there
may not be an easy way to interrupt the thread or where letting the
thread die in the middle of its work does not lose or corrupt data
(for example, a thread that generates “heart beats” for a service
monitoring tool). To mark a thread as a daemon, call its
<tt class="xref py py-func docutils literal"><span class="pre">setDaemon()</span></tt> method with a boolean argument. The default is for
threads to not be daemons, so passing True turns the daemon mode on.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'(</span><span class="si">%(threadName)-10s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">def</span> <span class="nf">daemon</span><span class="p">():</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Starting'</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Exiting'</span><span class="p">)</span>
<span class="n">d</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'daemon'</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">daemon</span><span class="p">)</span>
<span class="n">d</span><span class="o">.</span><span class="n">setDaemon</span><span class="p">(</span><span class="bp">True</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">non_daemon</span><span class="p">():</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Starting'</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Exiting'</span><span class="p">)</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'non-daemon'</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">non_daemon</span><span class="p">)</span>
<span class="n">d</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
</pre></div>
</div>
<p>Notice that the output does not include the <tt class="docutils literal"><span class="pre">"Exiting"</span></tt> message from
the daemon thread, since all of the non-daemon threads (including the
main thread) exit before the daemon thread wakes up from its two second
sleep.</p>
<div class="highlight-python"><pre>$ python threading_daemon.py
(daemon ) Starting
(non-daemon) Starting
(non-daemon) Exiting</pre>
</div>
<p>To wait until a daemon thread has completed its work, use the
<tt class="xref py py-func docutils literal"><span class="pre">join()</span></tt> method.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'(</span><span class="si">%(threadName)-10s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">def</span> <span class="nf">daemon</span><span class="p">():</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Starting'</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Exiting'</span><span class="p">)</span>
<span class="n">d</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'daemon'</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">daemon</span><span class="p">)</span>
<span class="n">d</span><span class="o">.</span><span class="n">setDaemon</span><span class="p">(</span><span class="bp">True</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">non_daemon</span><span class="p">():</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Starting'</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Exiting'</span><span class="p">)</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'non-daemon'</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">non_daemon</span><span class="p">)</span>
<span class="n">d</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">d</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
<span class="n">t</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
</pre></div>
</div>
<p>Waiting for the daemon thread to exit using <tt class="xref py py-func docutils literal"><span class="pre">join()</span></tt> means it
has a chance to produce its <tt class="docutils literal"><span class="pre">"Exiting"</span></tt> message.</p>
<div class="highlight-python"><pre>$ python threading_daemon_join.py
(daemon ) Starting
(non-daemon) Starting
(non-daemon) Exiting
(daemon ) Exiting</pre>
</div>
<p>By default, <tt class="xref py py-func docutils literal"><span class="pre">join()</span></tt> blocks indefinitely. It is also possible to
pass a timeout argument (a float representing the number of seconds to
wait for the thread to become inactive). If the thread does not
complete within the timeout period, <tt class="xref py py-func docutils literal"><span class="pre">join()</span></tt> returns anyway.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'(</span><span class="si">%(threadName)-10s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">def</span> <span class="nf">daemon</span><span class="p">():</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Starting'</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Exiting'</span><span class="p">)</span>
<span class="n">d</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'daemon'</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">daemon</span><span class="p">)</span>
<span class="n">d</span><span class="o">.</span><span class="n">setDaemon</span><span class="p">(</span><span class="bp">True</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">non_daemon</span><span class="p">():</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Starting'</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Exiting'</span><span class="p">)</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'non-daemon'</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">non_daemon</span><span class="p">)</span>
<span class="n">d</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">d</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'d.isAlive()'</span><span class="p">,</span> <span class="n">d</span><span class="o">.</span><span class="n">isAlive</span><span class="p">()</span>
<span class="n">t</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
</pre></div>
</div>
<p>Since the timeout passed is less than the amount of time the daemon
thread sleeps, the thread is still “alive” after <tt class="xref py py-func docutils literal"><span class="pre">join()</span></tt>
returns.</p>
<div class="highlight-python"><pre>$ python threading_daemon_join_timeout.py
(daemon ) Starting
(non-daemon) Starting
(non-daemon) Exiting
d.isAlive() True</pre>
</div>
</div>
<div class="section" id="enumerating-all-threads">
<h2>Enumerating All Threads<a class="headerlink" href="#enumerating-all-threads" title="Permalink to this headline">¶</a></h2>
<p>It is not necessary to retain an explicit handle to all of the daemon
threads in order to ensure they have completed before exiting the main
process. <tt class="xref py py-func docutils literal"><span class="pre">enumerate()</span></tt> returns a list of active <tt class="xref py py-class docutils literal"><span class="pre">Thread</span></tt>
instances. The list includes the current thread, and since joining the
current thread is not allowed (it introduces a deadlock situation), it
must be skipped.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">random</span>
<span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'(</span><span class="si">%(threadName)-10s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">def</span> <span class="nf">worker</span><span class="p">():</span>
<span class="sd">"""thread worker function"""</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">currentThread</span><span class="p">()</span>
<span class="n">pause</span> <span class="o">=</span> <span class="n">random</span><span class="o">.</span><span class="n">randint</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">5</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'sleeping </span><span class="si">%s</span><span class="s">'</span><span class="p">,</span> <span class="n">pause</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="n">pause</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'ending'</span><span class="p">)</span>
<span class="k">return</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">):</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">)</span>
<span class="n">t</span><span class="o">.</span><span class="n">setDaemon</span><span class="p">(</span><span class="bp">True</span><span class="p">)</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">main_thread</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">currentThread</span><span class="p">()</span>
<span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">threading</span><span class="o">.</span><span class="n">enumerate</span><span class="p">():</span>
<span class="k">if</span> <span class="n">t</span> <span class="ow">is</span> <span class="n">main_thread</span><span class="p">:</span>
<span class="k">continue</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'joining </span><span class="si">%s</span><span class="s">'</span><span class="p">,</span> <span class="n">t</span><span class="o">.</span><span class="n">getName</span><span class="p">())</span>
<span class="n">t</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
</pre></div>
</div>
<p>Since the worker is sleeping for a random amount of time, the output
from this program may vary. It should look something like this:</p>
<div class="highlight-python"><pre>$ python threading_enumerate.py
(Thread-1 ) sleeping 1
(Thread-2 ) sleeping 2
(Thread-3 ) sleeping 3
(MainThread) joining Thread-1
(Thread-1 ) ending
(MainThread) joining Thread-2
(Thread-2 ) ending
(MainThread) joining Thread-3
(Thread-3 ) ending</pre>
</div>
</div>
<div class="section" id="subclassing-thread">
<h2>Subclassing Thread<a class="headerlink" href="#subclassing-thread" title="Permalink to this headline">¶</a></h2>
<p>At start-up, a <tt class="xref py py-class docutils literal"><span class="pre">Thread</span></tt> does some basic initialization and then
calls its <tt class="xref py py-func docutils literal"><span class="pre">run()</span></tt> method, which calls the target function passed
to the constructor. To create a subclass of <tt class="xref py py-class docutils literal"><span class="pre">Thread</span></tt>, override
<tt class="xref py py-func docutils literal"><span class="pre">run()</span></tt> to do whatever is necessary.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'(</span><span class="si">%(threadName)-10s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">class</span> <span class="nc">MyThread</span><span class="p">(</span><span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">run</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'running'</span><span class="p">)</span>
<span class="k">return</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</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">MyThread</span><span class="p">()</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
</pre></div>
</div>
<p>The return value of <tt class="xref py py-func docutils literal"><span class="pre">run()</span></tt> is ignored.</p>
<div class="highlight-python"><pre>$ python threading_subclass.py
(Thread-1 ) running
(Thread-2 ) running
(Thread-3 ) running
(Thread-4 ) running
(Thread-5 ) running</pre>
</div>
<p>Because the <em>args</em> and <em>kwargs</em> values passed to the <tt class="xref py py-class docutils literal"><span class="pre">Thread</span></tt>
constructor are saved in private variables, they are not easily
accessed from a subclass. To pass arguments to a custom thread type,
redefine the constructor to save the values in an instance attribute
that can be seen in the subclass.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'(</span><span class="si">%(threadName)-10s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">class</span> <span class="nc">MyThreadWithArgs</span><span class="p">(</span><span class="n">threading</span><span class="o">.</span><span class="n">Thread</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">group</span><span class="o">=</span><span class="bp">None</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="bp">None</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="bp">None</span><span class="p">,</span>
<span class="n">args</span><span class="o">=</span><span class="p">(),</span> <span class="n">kwargs</span><span class="o">=</span><span class="bp">None</span><span class="p">,</span> <span class="n">verbose</span><span class="o">=</span><span class="bp">None</span><span class="p">):</span>
<span class="n">threading</span><span class="o">.</span><span class="n">Thread</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">group</span><span class="o">=</span><span class="n">group</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">target</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="n">name</span><span class="p">,</span>
<span class="n">verbose</span><span class="o">=</span><span class="n">verbose</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">args</span> <span class="o">=</span> <span class="n">args</span>
<span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span> <span class="o">=</span> <span class="n">kwargs</span>
<span class="k">return</span>
<span class="k">def</span> <span class="nf">run</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'running with </span><span class="si">%s</span><span class="s"> and </span><span class="si">%s</span><span class="s">'</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">kwargs</span><span class="p">)</span>
<span class="k">return</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</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">MyThreadWithArgs</span><span class="p">(</span><span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">i</span><span class="p">,),</span> <span class="n">kwargs</span><span class="o">=</span><span class="p">{</span><span class="s">'a'</span><span class="p">:</span><span class="s">'A'</span><span class="p">,</span> <span class="s">'b'</span><span class="p">:</span><span class="s">'B'</span><span class="p">})</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
</pre></div>
</div>
<p><tt class="xref py py-class docutils literal"><span class="pre">MyThreadWithArgs</span></tt> uses the same API as <tt class="xref py py-class docutils literal"><span class="pre">Thread</span></tt>, but
another class could easily change the constructor method to take more
or different arguments more directly related to the purpose of the
thread, as with any other class.</p>
<div class="highlight-python"><pre>$ python threading_subclass_args.py
(Thread-1 ) running with (0,) and {'a': 'A', 'b': 'B'}
(Thread-2 ) running with (1,) and {'a': 'A', 'b': 'B'}
(Thread-3 ) running with (2,) and {'a': 'A', 'b': 'B'}
(Thread-4 ) running with (3,) and {'a': 'A', 'b': 'B'}
(Thread-5 ) running with (4,) and {'a': 'A', 'b': 'B'}</pre>
</div>
</div>
<div class="section" id="timer-threads">
<h2>Timer Threads<a class="headerlink" href="#timer-threads" title="Permalink to this headline">¶</a></h2>
<p>One example of a reason to subclass <tt class="xref py py-class docutils literal"><span class="pre">Thread</span></tt> is provided by
<tt class="xref py py-class docutils literal"><span class="pre">Timer</span></tt>, also included in <a class="reference internal" href="#module-threading" title="threading: Manage several concurrent threads of execution."><tt class="xref py py-mod docutils literal"><span class="pre">threading</span></tt></a>. A <tt class="xref py py-class docutils literal"><span class="pre">Timer</span></tt>
starts its work after a delay, and can be canceled at any point within
that delay time period.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'(</span><span class="si">%(threadName)-10s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">def</span> <span class="nf">delayed</span><span class="p">():</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'worker running'</span><span class="p">)</span>
<span class="k">return</span>
<span class="n">t1</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Timer</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="n">delayed</span><span class="p">)</span>
<span class="n">t1</span><span class="o">.</span><span class="n">setName</span><span class="p">(</span><span class="s">'t1'</span><span class="p">)</span>
<span class="n">t2</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Timer</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="n">delayed</span><span class="p">)</span>
<span class="n">t2</span><span class="o">.</span><span class="n">setName</span><span class="p">(</span><span class="s">'t2'</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'starting timers'</span><span class="p">)</span>
<span class="n">t1</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">t2</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'waiting before canceling </span><span class="si">%s</span><span class="s">'</span><span class="p">,</span> <span class="n">t2</span><span class="o">.</span><span class="n">getName</span><span class="p">())</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'canceling </span><span class="si">%s</span><span class="s">'</span><span class="p">,</span> <span class="n">t2</span><span class="o">.</span><span class="n">getName</span><span class="p">())</span>
<span class="n">t2</span><span class="o">.</span><span class="n">cancel</span><span class="p">()</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'done'</span><span class="p">)</span>
</pre></div>
</div>
<p>Notice that the second timer is never run, and the first timer appears
to run after the rest of the main program is done. Since it is not a
daemon thread, it is joined implicitly when the main thread is done.</p>
<div class="highlight-python"><pre>$ python threading_timer.py
(MainThread) starting timers
(MainThread) waiting before canceling t2
(MainThread) canceling t2
(MainThread) done
(t1 ) worker running</pre>
</div>
</div>
<div class="section" id="signaling-between-threads">
<h2>Signaling Between Threads<a class="headerlink" href="#signaling-between-threads" title="Permalink to this headline">¶</a></h2>
<p>Although the point of using multiple threads is to spin separate
operations off to run concurrently, there are times when it is
important to be able to synchronize the operations in two or more
threads. A simple way to communicate between threads is using
<tt class="xref py py-class docutils literal"><span class="pre">Event</span></tt> objects. An <tt class="xref py py-class docutils literal"><span class="pre">Event</span></tt> manages an internal flag
that callers can either <tt class="xref py py-func docutils literal"><span class="pre">set()</span></tt> or <tt class="xref py py-func docutils literal"><span class="pre">clear()</span></tt>. Other
threads can <tt class="xref py py-func docutils literal"><span class="pre">wait()</span></tt> for the flag to be <tt class="xref py py-func docutils literal"><span class="pre">set()</span></tt>,
effectively blocking progress until allowed to continue.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'(</span><span class="si">%(threadName)-10s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">def</span> <span class="nf">wait_for_event</span><span class="p">(</span><span class="n">e</span><span class="p">):</span>
<span class="sd">"""Wait for the event to be set before doing anything"""</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'wait_for_event starting'</span><span class="p">)</span>
<span class="n">event_is_set</span> <span class="o">=</span> <span class="n">e</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'event set: </span><span class="si">%s</span><span class="s">'</span><span class="p">,</span> <span class="n">event_is_set</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">wait_for_event_timeout</span><span class="p">(</span><span class="n">e</span><span class="p">,</span> <span class="n">t</span><span class="p">):</span>
<span class="sd">"""Wait t seconds and then timeout"""</span>
<span class="k">while</span> <span class="ow">not</span> <span class="n">e</span><span class="o">.</span><span class="n">isSet</span><span class="p">():</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'wait_for_event_timeout starting'</span><span class="p">)</span>
<span class="n">event_is_set</span> <span class="o">=</span> <span class="n">e</span><span class="o">.</span><span class="n">wait</span><span class="p">(</span><span class="n">t</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'event set: </span><span class="si">%s</span><span class="s">'</span><span class="p">,</span> <span class="n">event_is_set</span><span class="p">)</span>
<span class="k">if</span> <span class="n">event_is_set</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'processing event'</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'doing other work'</span><span class="p">)</span>
<span class="n">e</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Event</span><span class="p">()</span>
<span class="n">t1</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'block'</span><span class="p">,</span>
<span class="n">target</span><span class="o">=</span><span class="n">wait_for_event</span><span class="p">,</span>
<span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">e</span><span class="p">,))</span>
<span class="n">t1</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">t2</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'non-block'</span><span class="p">,</span>
<span class="n">target</span><span class="o">=</span><span class="n">wait_for_event_timeout</span><span class="p">,</span>
<span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">e</span><span class="p">,</span> <span class="mi">2</span><span class="p">))</span>
<span class="n">t2</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Waiting before calling Event.set()'</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>
<span class="n">e</span><span class="o">.</span><span class="n">set</span><span class="p">()</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Event is set'</span><span class="p">)</span>
</pre></div>
</div>
<p>The <tt class="xref py py-func docutils literal"><span class="pre">wait()</span></tt> method takes an argument representing the number of
seconds to wait for the event before timing out. It returns a boolean
indicating whether or not the event is set, so the caller knows why
<tt class="xref py py-func docutils literal"><span class="pre">wait()</span></tt> returned. The <tt class="xref py py-func docutils literal"><span class="pre">isSet()</span></tt> method can be used
separately on the event without fear of blocking.</p>
<p>In this example, <tt class="xref py py-func docutils literal"><span class="pre">wait_for_event_timeout()</span></tt> checks the event
status without blocking indefinitely. The <tt class="xref py py-func docutils literal"><span class="pre">wait_for_event()</span></tt>
blocks on the call to <tt class="xref py py-func docutils literal"><span class="pre">wait()</span></tt>, which does not return until the
event status changes.</p>
<div class="highlight-python"><pre>$ python threading_event.py
(block ) wait_for_event starting
(non-block ) wait_for_event_timeout starting
(MainThread) Waiting before calling Event.set()
(non-block ) event set: False
(non-block ) doing other work
(non-block ) wait_for_event_timeout starting
(MainThread) Event is set
(block ) event set: True
(non-block ) event set: True
(non-block ) processing event</pre>
</div>
</div>
<div class="section" id="controlling-access-to-resources">
<h2>Controlling Access to Resources<a class="headerlink" href="#controlling-access-to-resources" title="Permalink to this headline">¶</a></h2>
<p>In addition to synchronizing the operations of threads, it is also
important to be able to control access to shared resources to prevent
corruption or missed data. Python’s built-in data structures (lists,
dictionaries, etc.) are thread-safe as a side-effect of having atomic
byte-codes for manipulating them (the GIL is not released in the
middle of an update). Other data structures implemented in Python, or
simpler types like integers and floats, don’t have that protection. To
guard against simultaneous access to an object, use a <tt class="xref py py-class docutils literal"><span class="pre">Lock</span></tt>
object.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">random</span>
<span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'(</span><span class="si">%(threadName)-10s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">class</span> <span class="nc">Counter</span><span class="p">(</span><span class="nb">object</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">start</span><span class="o">=</span><span class="mi">0</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">lock</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Lock</span><span class="p">()</span>
<span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="n">start</span>
<span class="k">def</span> <span class="nf">increment</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Waiting for lock'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">lock</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Acquired lock'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">+</span> <span class="mi">1</span>
<span class="k">finally</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">lock</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">worker</span><span class="p">(</span><span class="n">c</span><span class="p">):</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">2</span><span class="p">):</span>
<span class="n">pause</span> <span class="o">=</span> <span class="n">random</span><span class="o">.</span><span class="n">random</span><span class="p">()</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Sleeping </span><span class="si">%0.02f</span><span class="s">'</span><span class="p">,</span> <span class="n">pause</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="n">pause</span><span class="p">)</span>
<span class="n">c</span><span class="o">.</span><span class="n">increment</span><span class="p">()</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Done'</span><span class="p">)</span>
<span class="n">counter</span> <span class="o">=</span> <span class="n">Counter</span><span class="p">()</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">2</span><span class="p">):</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">counter</span><span class="p">,))</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Waiting for worker threads'</span><span class="p">)</span>
<span class="n">main_thread</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">currentThread</span><span class="p">()</span>
<span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">threading</span><span class="o">.</span><span class="n">enumerate</span><span class="p">():</span>
<span class="k">if</span> <span class="n">t</span> <span class="ow">is</span> <span class="ow">not</span> <span class="n">main_thread</span><span class="p">:</span>
<span class="n">t</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Counter: </span><span class="si">%d</span><span class="s">'</span><span class="p">,</span> <span class="n">counter</span><span class="o">.</span><span class="n">value</span><span class="p">)</span>
</pre></div>
</div>
<p>In this example, the <tt class="xref py py-func docutils literal"><span class="pre">worker()</span></tt> function increments a
<tt class="xref py py-class docutils literal"><span class="pre">Counter</span></tt> instance, which manages a <tt class="xref py py-class docutils literal"><span class="pre">Lock</span></tt> to prevent
two threads from changing its internal state at the same time. If the
<tt class="xref py py-class docutils literal"><span class="pre">Lock</span></tt> was not used, there is a possibility of missing a change
to the value attribute.</p>
<div class="highlight-python"><pre>$ python threading_lock.py
(Thread-1 ) Sleeping 0.04
(Thread-2 ) Sleeping 0.47
(MainThread) Waiting for worker threads
(Thread-1 ) Waiting for lock
(Thread-1 ) Acquired lock
(Thread-1 ) Sleeping 0.33
(Thread-1 ) Waiting for lock
(Thread-1 ) Acquired lock
(Thread-1 ) Done
(Thread-2 ) Waiting for lock
(Thread-2 ) Acquired lock
(Thread-2 ) Sleeping 0.41
(Thread-2 ) Waiting for lock
(Thread-2 ) Acquired lock
(Thread-2 ) Done
(MainThread) Counter: 4</pre>
</div>
<p>To find out whether another thread has acquired the lock without
holding up the current thread, pass False for the <em>blocking</em> argument
to <tt class="xref py py-func docutils literal"><span class="pre">acquire()</span></tt>. In the next example, <tt class="xref py py-func docutils literal"><span class="pre">worker()</span></tt> tries to
acquire the lock three separate times, and counts how many attempts it
has to make to do so. In the mean time, <tt class="xref py py-func docutils literal"><span class="pre">lock_holder()</span></tt> cycles
between holding and releasing the lock, with short pauses in each
state used to simulate load.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'(</span><span class="si">%(threadName)-10s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">def</span> <span class="nf">lock_holder</span><span class="p">(</span><span class="n">lock</span><span class="p">):</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Starting'</span><span class="p">)</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="n">lock</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Holding'</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.5</span><span class="p">)</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Not holding'</span><span class="p">)</span>
<span class="n">lock</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.5</span><span class="p">)</span>
<span class="k">return</span>
<span class="k">def</span> <span class="nf">worker</span><span class="p">(</span><span class="n">lock</span><span class="p">):</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Starting'</span><span class="p">)</span>
<span class="n">num_tries</span> <span class="o">=</span> <span class="mi">0</span>
<span class="n">num_acquires</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">while</span> <span class="n">num_acquires</span> <span class="o"><</span> <span class="mi">3</span><span class="p">:</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.5</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Trying to acquire'</span><span class="p">)</span>
<span class="n">have_it</span> <span class="o">=</span> <span class="n">lock</span><span class="o">.</span><span class="n">acquire</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">num_tries</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="k">if</span> <span class="n">have_it</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Iteration </span><span class="si">%d</span><span class="s">: Acquired'</span><span class="p">,</span> <span class="n">num_tries</span><span class="p">)</span>
<span class="n">num_acquires</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Iteration </span><span class="si">%d</span><span class="s">: Not acquired'</span><span class="p">,</span> <span class="n">num_tries</span><span class="p">)</span>
<span class="k">finally</span><span class="p">:</span>
<span class="k">if</span> <span class="n">have_it</span><span class="p">:</span>
<span class="n">lock</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Done after </span><span class="si">%d</span><span class="s"> iterations'</span><span class="p">,</span> <span class="n">num_tries</span><span class="p">)</span>
<span class="n">lock</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Lock</span><span class="p">()</span>
<span class="n">holder</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">lock_holder</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">lock</span><span class="p">,),</span> <span class="n">name</span><span class="o">=</span><span class="s">'LockHolder'</span><span class="p">)</span>
<span class="n">holder</span><span class="o">.</span><span class="n">setDaemon</span><span class="p">(</span><span class="bp">True</span><span class="p">)</span>
<span class="n">holder</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">worker</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">lock</span><span class="p">,),</span> <span class="n">name</span><span class="o">=</span><span class="s">'Worker'</span><span class="p">)</span>
<span class="n">worker</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
</pre></div>
</div>
<p>It takes <tt class="xref py py-func docutils literal"><span class="pre">worker()</span></tt> more than three iterations to acquire the lock
three separate times.</p>
<div class="highlight-python"><pre>$ python threading_lock_noblock.py
(LockHolder) Starting
(LockHolder) Holding
(Worker ) Starting
(LockHolder) Not holding
(Worker ) Trying to acquire
(Worker ) Iteration 1: Acquired
(LockHolder) Holding
(Worker ) Trying to acquire
(Worker ) Iteration 2: Not acquired
(LockHolder) Not holding
(Worker ) Trying to acquire
(Worker ) Iteration 3: Acquired
(LockHolder) Holding
(Worker ) Trying to acquire
(Worker ) Iteration 4: Not acquired
(LockHolder) Not holding
(Worker ) Trying to acquire
(Worker ) Iteration 5: Acquired
(Worker ) Done after 5 iterations</pre>
</div>
<div class="section" id="re-entrant-locks">
<h3>Re-entrant Locks<a class="headerlink" href="#re-entrant-locks" title="Permalink to this headline">¶</a></h3>
<p>Normal <tt class="xref py py-class docutils literal"><span class="pre">Lock</span></tt> objects cannot be acquired more than once, even
by the same thread. This can introduce undesirable side-effects if a
lock is accessed by more than one function in the same call chain.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">threading</span>
<span class="n">lock</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Lock</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'First try :'</span><span class="p">,</span> <span class="n">lock</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'Second try:'</span><span class="p">,</span> <span class="n">lock</span><span class="o">.</span><span class="n">acquire</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
</pre></div>
</div>
<p>In this case, since both functions are using the same global lock, and
one calls the other, the second acquisition fails and would have
blocked using the default arguments to <tt class="xref py py-func docutils literal"><span class="pre">acquire()</span></tt>.</p>
<div class="highlight-python"><pre>$ python threading_lock_reacquire.py
First try : True
Second try: False</pre>
</div>
<p>In a situation where separate code from the same thread needs to
“re-acquire” the lock, use an <tt class="xref py py-class docutils literal"><span class="pre">RLock</span></tt> instead.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">threading</span>
<span class="n">lock</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">RLock</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'First try :'</span><span class="p">,</span> <span class="n">lock</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>
<span class="k">print</span> <span class="s">'Second try:'</span><span class="p">,</span> <span class="n">lock</span><span class="o">.</span><span class="n">acquire</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
</pre></div>
</div>
<p>The only change to the code from the previous example was substituting
<tt class="xref py py-class docutils literal"><span class="pre">RLock</span></tt> for <tt class="xref py py-class docutils literal"><span class="pre">Lock</span></tt>.</p>
<div class="highlight-python"><pre>$ python threading_rlock.py
First try : True
Second try: 1</pre>
</div>
</div>
<div class="section" id="locks-as-context-managers">
<h3>Locks as Context Managers<a class="headerlink" href="#locks-as-context-managers" title="Permalink to this headline">¶</a></h3>
<p>Locks implement the context manager API and are compatible with the
<strong class="command">with</strong> statement. Using <strong class="command">with</strong> removes the need to
explicitly acquire and release the lock.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'(</span><span class="si">%(threadName)-10s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">def</span> <span class="nf">worker_with</span><span class="p">(</span><span class="n">lock</span><span class="p">):</span>
<span class="k">with</span> <span class="n">lock</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Lock acquired via with'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">worker_no_with</span><span class="p">(</span><span class="n">lock</span><span class="p">):</span>
<span class="n">lock</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Lock acquired directly'</span><span class="p">)</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">lock</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>
<span class="n">lock</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Lock</span><span class="p">()</span>
<span class="n">w</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker_with</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">lock</span><span class="p">,))</span>
<span class="n">nw</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker_no_with</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">lock</span><span class="p">,))</span>
<span class="n">w</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">nw</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
</pre></div>
</div>
<p>The two functions <tt class="xref py py-func docutils literal"><span class="pre">worker_with()</span></tt> and <tt class="xref py py-func docutils literal"><span class="pre">worker_no_with()</span></tt>
manage the lock in equivalent ways.</p>
<div class="highlight-python"><pre>$ python threading_lock_with.py
(Thread-1 ) Lock acquired via with
(Thread-2 ) Lock acquired directly</pre>
</div>
</div>
</div>
<div class="section" id="synchronizing-threads">
<h2>Synchronizing Threads<a class="headerlink" href="#synchronizing-threads" title="Permalink to this headline">¶</a></h2>
<p>In addition to using <tt class="xref py py-class docutils literal"><span class="pre">Events</span></tt>, another way of synchronizing
threads is through using a <tt class="xref py py-class docutils literal"><span class="pre">Condition</span></tt> object. Because the
<tt class="xref py py-class docutils literal"><span class="pre">Condition</span></tt> uses a <tt class="xref py py-class docutils literal"><span class="pre">Lock</span></tt>, it can be tied to a shared
resource. This allows threads to wait for the resource to be updated.
In this example, the <tt class="xref py py-func docutils literal"><span class="pre">consumer()</span></tt> threads <tt class="xref py py-func docutils literal"><span class="pre">wait()</span></tt> for the
<tt class="xref py py-class docutils literal"><span class="pre">Condition</span></tt> to be set before continuing. The <tt class="xref py py-func docutils literal"><span class="pre">producer()</span></tt>
thread is responsible for setting the condition and notifying the
other threads that they can continue.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'</span><span class="si">%(asctime)s</span><span class="s"> (</span><span class="si">%(threadName)-2s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">def</span> <span class="nf">consumer</span><span class="p">(</span><span class="n">cond</span><span class="p">):</span>
<span class="sd">"""wait for the condition and use the resource"""</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Starting consumer thread'</span><span class="p">)</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">currentThread</span><span class="p">()</span>
<span class="k">with</span> <span class="n">cond</span><span class="p">:</span>
<span class="n">cond</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Resource is available to consumer'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">producer</span><span class="p">(</span><span class="n">cond</span><span class="p">):</span>
<span class="sd">"""set up the resource to be used by the consumer"""</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Starting producer thread'</span><span class="p">)</span>
<span class="k">with</span> <span class="n">cond</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Making resource available'</span><span class="p">)</span>
<span class="n">cond</span><span class="o">.</span><span class="n">notifyAll</span><span class="p">()</span>
<span class="n">condition</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Condition</span><span class="p">()</span>
<span class="n">c1</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'c1'</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">consumer</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">condition</span><span class="p">,))</span>
<span class="n">c2</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'c2'</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">consumer</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">condition</span><span class="p">,))</span>
<span class="n">p</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'p'</span><span class="p">,</span> <span class="n">target</span><span class="o">=</span><span class="n">producer</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">condition</span><span class="p">,))</span>
<span class="n">c1</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="n">c2</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="n">p</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
</pre></div>
</div>
<p>The threads use <strong class="command">with</strong> to acquire the lock associated with
the <tt class="xref py py-class docutils literal"><span class="pre">Condition</span></tt>. Using the <tt class="xref py py-func docutils literal"><span class="pre">acquire()</span></tt> and
<tt class="xref py py-func docutils literal"><span class="pre">release()</span></tt> methods explicitly also works.</p>
<div class="highlight-python"><pre>$ python threading_condition.py
2010-10-24 08:55:41,008 (c1) Starting consumer thread
2010-10-24 08:55:43,009 (c2) Starting consumer thread
2010-10-24 08:55:45,009 (p ) Starting producer thread
2010-10-24 08:55:45,010 (p ) Making resource available
2010-10-24 08:55:45,010 (c1) Resource is available to consumer
2010-10-24 08:55:45,011 (c2) Resource is available to consumer</pre>
</div>
</div>
<div class="section" id="limiting-concurrent-access-to-resources">
<h2>Limiting Concurrent Access to Resources<a class="headerlink" href="#limiting-concurrent-access-to-resources" title="Permalink to this headline">¶</a></h2>
<p>Sometimes it is useful to allow more than one worker access to a
resource at a time, while still limiting the overall number. For
example, a connection pool might support a fixed number of
simultaneous connections, or a network application might support a
fixed number of concurrent downloads. A <tt class="xref py py-class docutils literal"><span class="pre">Semaphore</span></tt> is one way
to manage those connections.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">logging</span>
<span class="kn">import</span> <span class="nn">random</span>
<span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'</span><span class="si">%(asctime)s</span><span class="s"> (</span><span class="si">%(threadName)-2s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">class</span> <span class="nc">ActivePool</span><span class="p">(</span><span class="nb">object</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="nb">super</span><span class="p">(</span><span class="n">ActivePool</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">__init__</span><span class="p">()</span>
<span class="bp">self</span><span class="o">.</span><span class="n">active</span> <span class="o">=</span> <span class="p">[]</span>
<span class="bp">self</span><span class="o">.</span><span class="n">lock</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Lock</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">makeActive</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="k">with</span> <span class="bp">self</span><span class="o">.</span><span class="n">lock</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">active</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Running: </span><span class="si">%s</span><span class="s">'</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">active</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">makeInactive</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="k">with</span> <span class="bp">self</span><span class="o">.</span><span class="n">lock</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">active</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Running: </span><span class="si">%s</span><span class="s">'</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">active</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">worker</span><span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="n">pool</span><span class="p">):</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Waiting to join the pool'</span><span class="p">)</span>
<span class="k">with</span> <span class="n">s</span><span class="p">:</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">currentThread</span><span class="p">()</span><span class="o">.</span><span class="n">getName</span><span class="p">()</span>
<span class="n">pool</span><span class="o">.</span><span class="n">makeActive</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.1</span><span class="p">)</span>
<span class="n">pool</span><span class="o">.</span><span class="n">makeInactive</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
<span class="n">pool</span> <span class="o">=</span> <span class="n">ActivePool</span><span class="p">()</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Semaphore</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">):</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="nb">str</span><span class="p">(</span><span class="n">i</span><span class="p">),</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="n">pool</span><span class="p">))</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
</pre></div>
</div>
<p>In this example, the <tt class="xref py py-class docutils literal"><span class="pre">ActivePool</span></tt> class simply serves as a
convenient way to track which threads are able to run at a given
moment. A real resource pool would allocate a connection or some other
value to the newly active thread, and reclaim the value when the
thread is done. Here it is just used to hold the names of the active
threads to show that only five are running concurrently.</p>
<div class="highlight-python"><pre>$ python threading_semaphore.py
2010-10-24 08:55:45,096 (0 ) Waiting to join the pool
2010-10-24 08:55:45,097 (0 ) Running: ['0']
2010-10-24 08:55:45,097 (1 ) Waiting to join the pool
2010-10-24 08:55:45,097 (1 ) Running: ['0', '1']
2010-10-24 08:55:45,097 (2 ) Waiting to join the pool
2010-10-24 08:55:45,098 (3 ) Waiting to join the pool
2010-10-24 08:55:45,197 (0 ) Running: ['1']
2010-10-24 08:55:45,197 (2 ) Running: ['1', '2']
2010-10-24 08:55:45,197 (1 ) Running: ['2']
2010-10-24 08:55:45,198 (3 ) Running: ['2', '3']
2010-10-24 08:55:45,298 (2 ) Running: ['3']
2010-10-24 08:55:45,298 (3 ) Running: []</pre>
</div>
</div>
<div class="section" id="thread-specific-data">
<h2>Thread-specific Data<a class="headerlink" href="#thread-specific-data" title="Permalink to this headline">¶</a></h2>
<p>While some resources need to be locked so multiple threads can use
them, others need to be protected so that they are hidden from view in
threads that do not “own” them. The <tt class="xref py py-func docutils literal"><span class="pre">local()</span></tt> function creates
an object capable of hiding values from view in separate threads.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">random</span>
<span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'(</span><span class="si">%(threadName)-10s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">def</span> <span class="nf">show_value</span><span class="p">(</span><span class="n">data</span><span class="p">):</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">val</span> <span class="o">=</span> <span class="n">data</span><span class="o">.</span><span class="n">value</span>
<span class="k">except</span> <span class="ne">AttributeError</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'No value yet'</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'value=</span><span class="si">%s</span><span class="s">'</span><span class="p">,</span> <span class="n">val</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">worker</span><span class="p">(</span><span class="n">data</span><span class="p">):</span>
<span class="n">show_value</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="n">data</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="n">random</span><span class="o">.</span><span class="n">randint</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">100</span><span class="p">)</span>
<span class="n">show_value</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="n">local_data</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">local</span><span class="p">()</span>
<span class="n">show_value</span><span class="p">(</span><span class="n">local_data</span><span class="p">)</span>
<span class="n">local_data</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="mi">1000</span>
<span class="n">show_value</span><span class="p">(</span><span class="n">local_data</span><span class="p">)</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">2</span><span class="p">):</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">local_data</span><span class="p">,))</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
</pre></div>
</div>
<p>Notice that <tt class="docutils literal"><span class="pre">local_data.value</span></tt> is not present for any thread until
it is set in that thread.</p>
<div class="highlight-python"><pre>$ python threading_local.py
(MainThread) No value yet
(MainThread) value=1000
(Thread-1 ) No value yet
(Thread-1 ) value=19
(Thread-2 ) No value yet
(Thread-2 ) value=37</pre>
</div>
<p>To initialize the settings so all threads start with the same value,
use a subclass and set the attributes in <tt class="xref py py-func docutils literal"><span class="pre">__init__()</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">random</span>
<span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">logging</span>
<span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">DEBUG</span><span class="p">,</span>
<span class="n">format</span><span class="o">=</span><span class="s">'(</span><span class="si">%(threadName)-10s</span><span class="s">) </span><span class="si">%(message)s</span><span class="s">'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">def</span> <span class="nf">show_value</span><span class="p">(</span><span class="n">data</span><span class="p">):</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">val</span> <span class="o">=</span> <span class="n">data</span><span class="o">.</span><span class="n">value</span>
<span class="k">except</span> <span class="ne">AttributeError</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'No value yet'</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'value=</span><span class="si">%s</span><span class="s">'</span><span class="p">,</span> <span class="n">val</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">worker</span><span class="p">(</span><span class="n">data</span><span class="p">):</span>
<span class="n">show_value</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="n">data</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="n">random</span><span class="o">.</span><span class="n">randint</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">100</span><span class="p">)</span>
<span class="n">show_value</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">MyLocal</span><span class="p">(</span><span class="n">threading</span><span class="o">.</span><span class="n">local</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">value</span><span class="p">):</span>
<span class="n">logging</span><span class="o">.</span><span class="n">debug</span><span class="p">(</span><span class="s">'Initializing </span><span class="si">%r</span><span class="s">'</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="n">value</span>
<span class="n">local_data</span> <span class="o">=</span> <span class="n">MyLocal</span><span class="p">(</span><span class="mi">1000</span><span class="p">)</span>
<span class="n">show_value</span><span class="p">(</span><span class="n">local_data</span><span class="p">)</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">2</span><span class="p">):</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">local_data</span><span class="p">,))</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
</pre></div>
</div>
<p><tt class="xref py py-func docutils literal"><span class="pre">__init__()</span></tt> is invoked on the same object (note the <tt class="xref py py-func docutils literal"><span class="pre">id()</span></tt>
value), once in each thread.</p>
<div class="highlight-python"><pre>$ python threading_local_defaults.py
(MainThread) Initializing <__main__.MyLocal object at 0x1004e8100>
(MainThread) value=1000
(Thread-1 ) Initializing <__main__.MyLocal object at 0x1004e8100>
(Thread-1 ) value=1000
(Thread-1 ) value=39
(Thread-2 ) Initializing <__main__.MyLocal object at 0x1004e8100>
(Thread-2 ) value=1000
(Thread-2 ) value=5</pre>
</div>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><a class="reference external" href="http://docs.python.org/lib/module-threading.html">threading</a></dt>
<dd>Standard library documentation for this module.</dd>
<dt><tt class="xref py py-mod docutils literal"><span class="pre">thread</span></tt></dt>
<dd>Lower level thread API.</dd>
<dt><a class="reference internal" href="../Queue/index.html#module-Queue" title="Queue: Provides a thread-safe FIFO implementation"><tt class="xref py py-mod docutils literal"><span class="pre">Queue</span></tt></a></dt>
<dd>Thread-safe Queue, useful for passing messages between threads.</dd>
<dt><a class="reference internal" href="../multiprocessing/index.html#module-multiprocessing" title="multiprocessing: Manage processes like threads."><tt class="xref py py-mod docutils literal"><span class="pre">multiprocessing</span></tt></a></dt>
<dd>An API for working with processes that mirrors the <a class="reference internal" href="#module-threading" title="threading: Manage several concurrent threads of execution."><tt class="xref py py-mod docutils literal"><span class="pre">threading</span></tt></a> API.</dd>
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