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<div class="section" id="module-fpectl">
<span id="fpectl-floating-point-exception-control"></span><h1>27.16. <a class="reference internal" href="#module-fpectl" title="fpectl: Provide control for floating point exception handling. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">fpectl</span></tt></a> — Floating point exception control<a class="headerlink" href="#module-fpectl" title="Permalink to this headline">¶</a></h1>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The <a class="reference internal" href="#module-fpectl" title="fpectl: Provide control for floating point exception handling. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">fpectl</span></tt></a> module is not built by default, and its usage is discouraged
and may be dangerous except in the hands of experts. See also the section
<a class="reference internal" href="#fpectl-limitations"><em>Limitations and other considerations</em></a> on limitations for more details.</p>
</div>
<p id="index-0">Most computers carry out floating point operations in conformance with the
so-called IEEE-754 standard. On any real computer, some floating point
operations produce results that cannot be expressed as a normal floating point
value. For example, try</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">math</span>
<span class="gp">>>> </span><span class="n">math</span><span class="o">.</span><span class="n">exp</span><span class="p">(</span><span class="mi">1000</span><span class="p">)</span>
<span class="go">inf</span>
<span class="gp">>>> </span><span class="n">math</span><span class="o">.</span><span class="n">exp</span><span class="p">(</span><span class="mi">1000</span><span class="p">)</span> <span class="o">/</span> <span class="n">math</span><span class="o">.</span><span class="n">exp</span><span class="p">(</span><span class="mi">1000</span><span class="p">)</span>
<span class="go">nan</span>
</pre></div>
</div>
<p>(The example above will work on many platforms. DEC Alpha may be one exception.)
“Inf” is a special, non-numeric value in IEEE-754 that stands for “infinity”,
and “nan” means “not a number.” Note that, other than the non-numeric results,
nothing special happened when you asked Python to carry out those calculations.
That is in fact the default behaviour prescribed in the IEEE-754 standard, and
if it works for you, stop reading now.</p>
<p>In some circumstances, it would be better to raise an exception and stop
processing at the point where the faulty operation was attempted. The
<a class="reference internal" href="#module-fpectl" title="fpectl: Provide control for floating point exception handling. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">fpectl</span></tt></a> module is for use in that situation. It provides control over
floating point units from several hardware manufacturers, allowing the user to
turn on the generation of <tt class="xref py py-const docutils literal"><span class="pre">SIGFPE</span></tt> whenever any of the IEEE-754
exceptions Division by Zero, Overflow, or Invalid Operation occurs. In tandem
with a pair of wrapper macros that are inserted into the C code comprising your
python system, <tt class="xref py py-const docutils literal"><span class="pre">SIGFPE</span></tt> is trapped and converted into the Python
<a class="reference internal" href="#fpectl.FloatingPointError" title="fpectl.FloatingPointError"><tt class="xref py py-exc docutils literal"><span class="pre">FloatingPointError</span></tt></a> exception.</p>
<p>The <a class="reference internal" href="#module-fpectl" title="fpectl: Provide control for floating point exception handling. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">fpectl</span></tt></a> module defines the following functions and may raise the given
exception:</p>
<dl class="function">
<dt id="fpectl.turnon_sigfpe">
<tt class="descclassname">fpectl.</tt><tt class="descname">turnon_sigfpe</tt><big>(</big><big>)</big><a class="headerlink" href="#fpectl.turnon_sigfpe" title="Permalink to this definition">¶</a></dt>
<dd><p>Turn on the generation of <tt class="xref py py-const docutils literal"><span class="pre">SIGFPE</span></tt>, and set up an appropriate signal
handler.</p>
</dd></dl>
<dl class="function">
<dt id="fpectl.turnoff_sigfpe">
<tt class="descclassname">fpectl.</tt><tt class="descname">turnoff_sigfpe</tt><big>(</big><big>)</big><a class="headerlink" href="#fpectl.turnoff_sigfpe" title="Permalink to this definition">¶</a></dt>
<dd><p>Reset default handling of floating point exceptions.</p>
</dd></dl>
<dl class="exception">
<dt id="fpectl.FloatingPointError">
<em class="property">exception </em><tt class="descclassname">fpectl.</tt><tt class="descname">FloatingPointError</tt><a class="headerlink" href="#fpectl.FloatingPointError" title="Permalink to this definition">¶</a></dt>
<dd><p>After <a class="reference internal" href="#fpectl.turnon_sigfpe" title="fpectl.turnon_sigfpe"><tt class="xref py py-func docutils literal"><span class="pre">turnon_sigfpe()</span></tt></a> has been executed, a floating point operation that
raises one of the IEEE-754 exceptions Division by Zero, Overflow, or Invalid
operation will in turn raise this standard Python exception.</p>
</dd></dl>
<div class="section" id="example">
<span id="fpectl-example"></span><h2>27.16.1. Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h2>
<p>The following example demonstrates how to start up and test operation of the
<a class="reference internal" href="#module-fpectl" title="fpectl: Provide control for floating point exception handling. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">fpectl</span></tt></a> module.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">fpectl</span>
<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">fpetest</span>
<span class="gp">>>> </span><span class="n">fpectl</span><span class="o">.</span><span class="n">turnon_sigfpe</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">fpetest</span><span class="o">.</span><span class="n">test</span><span class="p">()</span>
<span class="go">overflow PASS</span>
<span class="go">FloatingPointError: Overflow</span>
<span class="go">div by 0 PASS</span>
<span class="go">FloatingPointError: Division by zero</span>
<span class="go"> [ more output from test elided ]</span>
<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">math</span>
<span class="gp">>>> </span><span class="n">math</span><span class="o">.</span><span class="n">exp</span><span class="p">(</span><span class="mi">1000</span><span class="p">)</span>
<span class="gt">Traceback (most recent call last):</span>
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n">?</span>
<span class="gr">FloatingPointError</span>: <span class="n">in math_1</span>
</pre></div>
</div>
</div>
<div class="section" id="limitations-and-other-considerations">
<span id="fpectl-limitations"></span><h2>27.16.2. Limitations and other considerations<a class="headerlink" href="#limitations-and-other-considerations" title="Permalink to this headline">¶</a></h2>
<p>Setting up a given processor to trap IEEE-754 floating point errors currently
requires custom code on a per-architecture basis. You may have to modify
<a class="reference internal" href="#module-fpectl" title="fpectl: Provide control for floating point exception handling. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">fpectl</span></tt></a> to control your particular hardware.</p>
<p>Conversion of an IEEE-754 exception to a Python exception requires that the
wrapper macros <tt class="docutils literal"><span class="pre">PyFPE_START_PROTECT</span></tt> and <tt class="docutils literal"><span class="pre">PyFPE_END_PROTECT</span></tt> be inserted
into your code in an appropriate fashion. Python itself has been modified to
support the <a class="reference internal" href="#module-fpectl" title="fpectl: Provide control for floating point exception handling. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">fpectl</span></tt></a> module, but many other codes of interest to numerical
analysts have not.</p>
<p>The <a class="reference internal" href="#module-fpectl" title="fpectl: Provide control for floating point exception handling. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">fpectl</span></tt></a> module is not thread-safe.</p>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last">Some files in the source distribution may be interesting in learning more about
how this module operates. The include file <a class="reference external" href="http://hg.python.org/cpython/file/2.7/Include/pyfpe.h">Include/pyfpe.h</a> discusses the
implementation of this module at some length. <a class="reference external" href="http://hg.python.org/cpython/file/2.7/Modules/fpetestmodule.c">Modules/fpetestmodule.c</a>
gives several examples of use. Many additional examples can be found in
<a class="reference external" href="http://hg.python.org/cpython/file/2.7/Objects/floatobject.c">Objects/floatobject.c</a>.</p>
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<ul>
<li><a class="reference internal" href="#">27.16. <tt class="docutils literal"><span class="pre">fpectl</span></tt> — Floating point exception control</a><ul>
<li><a class="reference internal" href="#example">27.16.1. Example</a></li>
<li><a class="reference internal" href="#limitations-and-other-considerations">27.16.2. Limitations and other considerations</a></li>
</ul>
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