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<div class="section" id="module-_thread">
<span id="thread-low-level-threading-api"></span><h1>16.9. <a class="reference internal" href="#module-_thread" title="_thread: Low-level threading API."><tt class="xref py py-mod docutils literal"><span class="pre">_thread</span></tt></a> — Low-level threading API<a class="headerlink" href="#module-_thread" title="Permalink to this headline">¶</a></h1>
<p id="index-0">This module provides low-level primitives for working with multiple threads
(also called <em class="dfn">light-weight processes</em> or <em class="dfn">tasks</em>) — multiple threads of
control sharing their global data space. For synchronization, simple locks
(also called <em class="dfn">mutexes</em> or <em class="dfn">binary semaphores</em>) are provided.
The <a class="reference internal" href="threading.html#module-threading" title="threading: Thread-based parallelism."><tt class="xref py py-mod docutils literal"><span class="pre">threading</span></tt></a> module provides an easier to use and higher-level
threading API built on top of this module.</p>
<p id="index-1">The module is optional. It is supported on Windows, Linux, SGI IRIX, Solaris
2.x, as well as on systems that have a POSIX thread (a.k.a. “pthread”)
implementation. For systems lacking the <a class="reference internal" href="#module-_thread" title="_thread: Low-level threading API."><tt class="xref py py-mod docutils literal"><span class="pre">_thread</span></tt></a> module, the
<a class="reference internal" href="_dummy_thread.html#module-_dummy_thread" title="_dummy_thread: Drop-in replacement for the _thread module."><tt class="xref py py-mod docutils literal"><span class="pre">_dummy_thread</span></tt></a> module is available. It duplicates this module’s interface
and can be used as a drop-in replacement.</p>
<p>It defines the following constants and functions:</p>
<dl class="exception">
<dt id="_thread.error">
<em class="property">exception </em><tt class="descclassname">_thread.</tt><tt class="descname">error</tt><a class="headerlink" href="#_thread.error" title="Permalink to this definition">¶</a></dt>
<dd><p>Raised on thread-specific errors.</p>
</dd></dl>
<dl class="data">
<dt id="_thread.LockType">
<tt class="descclassname">_thread.</tt><tt class="descname">LockType</tt><a class="headerlink" href="#_thread.LockType" title="Permalink to this definition">¶</a></dt>
<dd><p>This is the type of lock objects.</p>
</dd></dl>
<dl class="function">
<dt id="_thread.start_new_thread">
<tt class="descclassname">_thread.</tt><tt class="descname">start_new_thread</tt><big>(</big><em>function</em>, <em>args</em><span class="optional">[</span>, <em>kwargs</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#_thread.start_new_thread" title="Permalink to this definition">¶</a></dt>
<dd><p>Start a new thread and return its identifier. The thread executes the function
<em>function</em> with the argument list <em>args</em> (which must be a tuple). The optional
<em>kwargs</em> argument specifies a dictionary of keyword arguments. When the function
returns, the thread silently exits. When the function terminates with an
unhandled exception, a stack trace is printed and then the thread exits (but
other threads continue to run).</p>
</dd></dl>
<dl class="function">
<dt id="_thread.interrupt_main">
<tt class="descclassname">_thread.</tt><tt class="descname">interrupt_main</tt><big>(</big><big>)</big><a class="headerlink" href="#_thread.interrupt_main" title="Permalink to this definition">¶</a></dt>
<dd><p>Raise a <a class="reference internal" href="exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><tt class="xref py py-exc docutils literal"><span class="pre">KeyboardInterrupt</span></tt></a> exception in the main thread. A subthread can
use this function to interrupt the main thread.</p>
</dd></dl>
<dl class="function">
<dt id="_thread.exit">
<tt class="descclassname">_thread.</tt><tt class="descname">exit</tt><big>(</big><big>)</big><a class="headerlink" href="#_thread.exit" title="Permalink to this definition">¶</a></dt>
<dd><p>Raise the <a class="reference internal" href="exceptions.html#SystemExit" title="SystemExit"><tt class="xref py py-exc docutils literal"><span class="pre">SystemExit</span></tt></a> exception. When not caught, this will cause the
thread to exit silently.</p>
</dd></dl>
<dl class="function">
<dt id="_thread.allocate_lock">
<tt class="descclassname">_thread.</tt><tt class="descname">allocate_lock</tt><big>(</big><big>)</big><a class="headerlink" href="#_thread.allocate_lock" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a new lock object. Methods of locks are described below. The lock is
initially unlocked.</p>
</dd></dl>
<dl class="function">
<dt id="_thread.get_ident">
<tt class="descclassname">_thread.</tt><tt class="descname">get_ident</tt><big>(</big><big>)</big><a class="headerlink" href="#_thread.get_ident" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the ‘thread identifier’ of the current thread. This is a nonzero
integer. Its value has no direct meaning; it is intended as a magic cookie to
be used e.g. to index a dictionary of thread-specific data. Thread identifiers
may be recycled when a thread exits and another thread is created.</p>
</dd></dl>
<dl class="function">
<dt id="_thread.stack_size">
<tt class="descclassname">_thread.</tt><tt class="descname">stack_size</tt><big>(</big><span class="optional">[</span><em>size</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#_thread.stack_size" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the thread stack size used when creating new threads. The optional
<em>size</em> argument specifies the stack size to be used for subsequently created
threads, and must be 0 (use platform or configured default) or a positive
integer value of at least 32,768 (32kB). If changing the thread stack size is
unsupported, a <tt class="xref py py-exc docutils literal"><span class="pre">ThreadError</span></tt> is raised. If the specified stack size is
invalid, a <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><tt class="xref py py-exc docutils literal"><span class="pre">ValueError</span></tt></a> is raised and the stack size is unmodified. 32kB
is currently the minimum supported stack size value to guarantee sufficient
stack space for the interpreter itself. Note that some platforms may have
particular restrictions on values for the stack size, such as requiring a
minimum stack size > 32kB or requiring allocation in multiples of the system
memory page size - platform documentation should be referred to for more
information (4kB pages are common; using multiples of 4096 for the stack size is
the suggested approach in the absence of more specific information).
Availability: Windows, systems with POSIX threads.</p>
</dd></dl>
<dl class="data">
<dt id="_thread.TIMEOUT_MAX">
<tt class="descclassname">_thread.</tt><tt class="descname">TIMEOUT_MAX</tt><a class="headerlink" href="#_thread.TIMEOUT_MAX" title="Permalink to this definition">¶</a></dt>
<dd><p>The maximum value allowed for the <em>timeout</em> parameter of
<tt class="xref py py-meth docutils literal"><span class="pre">Lock.acquire()</span></tt>. Specifying a timeout greater than this value will
raise an <a class="reference internal" href="exceptions.html#OverflowError" title="OverflowError"><tt class="xref py py-exc docutils literal"><span class="pre">OverflowError</span></tt></a>.</p>
<p class="versionadded">
<span class="versionmodified">New in version 3.2.</span></p>
</dd></dl>
<p>Lock objects have the following methods:</p>
<dl class="method">
<dt id="_thread.lock.acquire">
<tt class="descclassname">lock.</tt><tt class="descname">acquire</tt><big>(</big><em>waitflag=1</em>, <em>timeout=-1</em><big>)</big><a class="headerlink" href="#_thread.lock.acquire" title="Permalink to this definition">¶</a></dt>
<dd><p>Without any optional argument, this method acquires the lock unconditionally, if
necessary waiting until it is released by another thread (only one thread at a
time can acquire a lock — that’s their reason for existence).</p>
<p>If the integer <em>waitflag</em> argument is present, the action depends on its
value: if it is zero, the lock is only acquired if it can be acquired
immediately without waiting, while if it is nonzero, the lock is acquired
unconditionally as above.</p>
<p>If the floating-point <em>timeout</em> argument is present and positive, it
specifies the maximum wait time in seconds before returning. A negative
<em>timeout</em> argument specifies an unbounded wait. You cannot specify
a <em>timeout</em> if <em>waitflag</em> is zero.</p>
<p>The return value is <tt class="xref docutils literal"><span class="pre">True</span></tt> if the lock is acquired successfully,
<tt class="xref docutils literal"><span class="pre">False</span></tt> if not.</p>
<p class="versionchanged">
<span class="versionmodified">Changed in version 3.2: </span>The <em>timeout</em> parameter is new.</p>
<p class="versionchanged">
<span class="versionmodified">Changed in version 3.2: </span>Lock acquires can now be interrupted by signals on POSIX.</p>
</dd></dl>
<dl class="method">
<dt id="_thread.lock.release">
<tt class="descclassname">lock.</tt><tt class="descname">release</tt><big>(</big><big>)</big><a class="headerlink" href="#_thread.lock.release" title="Permalink to this definition">¶</a></dt>
<dd><p>Releases the lock. The lock must have been acquired earlier, but not
necessarily by the same thread.</p>
</dd></dl>
<dl class="method">
<dt id="_thread.lock.locked">
<tt class="descclassname">lock.</tt><tt class="descname">locked</tt><big>(</big><big>)</big><a class="headerlink" href="#_thread.lock.locked" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the status of the lock: <tt class="xref docutils literal"><span class="pre">True</span></tt> if it has been acquired by some thread,
<tt class="xref docutils literal"><span class="pre">False</span></tt> if not.</p>
</dd></dl>
<p>In addition to these methods, lock objects can also be used via the
<a class="reference internal" href="../reference/compound_stmts.html#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a> statement, e.g.:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">_thread</span>
<span class="n">a_lock</span> <span class="o">=</span> <span class="n">_thread</span><span class="o">.</span><span class="n">allocate_lock</span><span class="p">()</span>
<span class="k">with</span> <span class="n">a_lock</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"a_lock is locked while this executes"</span><span class="p">)</span>
</pre></div>
</div>
<p><strong>Caveats:</strong></p>
<blockquote>
<div></div></blockquote>
<ul class="simple" id="index-2">
<li>Threads interact strangely with interrupts: the <a class="reference internal" href="exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><tt class="xref py py-exc docutils literal"><span class="pre">KeyboardInterrupt</span></tt></a>
exception will be received by an arbitrary thread. (When the <a class="reference internal" href="signal.html#module-signal" title="signal: Set handlers for asynchronous events."><tt class="xref py py-mod docutils literal"><span class="pre">signal</span></tt></a>
module is available, interrupts always go to the main thread.)</li>
<li>Calling <a class="reference internal" href="sys.html#sys.exit" title="sys.exit"><tt class="xref py py-func docutils literal"><span class="pre">sys.exit()</span></tt></a> or raising the <a class="reference internal" href="exceptions.html#SystemExit" title="SystemExit"><tt class="xref py py-exc docutils literal"><span class="pre">SystemExit</span></tt></a> exception is
equivalent to calling <a class="reference internal" href="#_thread.exit" title="_thread.exit"><tt class="xref py py-func docutils literal"><span class="pre">_thread.exit()</span></tt></a>.</li>
<li>Not all built-in functions that may block waiting for I/O allow other threads
to run. (The most popular ones (<a class="reference internal" href="time.html#time.sleep" title="time.sleep"><tt class="xref py py-func docutils literal"><span class="pre">time.sleep()</span></tt></a>, <tt class="xref py py-meth docutils literal"><span class="pre">file.read()</span></tt>,
<a class="reference internal" href="select.html#select.select" title="select.select"><tt class="xref py py-func docutils literal"><span class="pre">select.select()</span></tt></a>) work as expected.)</li>
<li>It is not possible to interrupt the <tt class="xref py py-meth docutils literal"><span class="pre">acquire()</span></tt> method on a lock — the
<a class="reference internal" href="exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><tt class="xref py py-exc docutils literal"><span class="pre">KeyboardInterrupt</span></tt></a> exception will happen after the lock has been acquired.</li>
<li>When the main thread exits, it is system defined whether the other threads
survive. On most systems, they are killed without executing
<a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> ... <a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clauses or executing object
destructors.</li>
<li>When the main thread exits, it does not do any of its usual cleanup (except
that <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> ... <a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clauses are honored), and the
standard I/O files are not flushed.</li>
</ul>
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