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<H1><A NAME="SECTION005400000000000000000">
3.4 <tt class="module">fpectl</tt> --
Floating point exception control</A>
</H1>
<P>
<p class="availability">Availability: <span
class="platform">Unix</span>.</p>
<P>
Most computers carry out floating point operations<a name="l2h-332"> </a>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>
<dl><dd><pre class="verbatim">
>>> import math
>>> math.exp(1000)
inf
>>> math.exp(1000) / math.exp(1000)
nan
</pre></dl>
<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>
In some circumstances,
it would be better to raise an exception and stop processing
at the point where the faulty operation was attempted.
The <tt class="module">fpectl</tt> 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="constant">SIGFPE</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="constant">SIGFPE</tt> is trapped and converted into the Python
<tt class="exception">FloatingPointError</tt> exception.
<P>
The <tt class="module">fpectl</tt> module defines the following functions and
may raise the given exception:
<P>
<dl><dt><b><a name="l2h-329"><tt class="function">turnon_sigfpe</tt></a></b>()
<dd>
Turn on the generation of <tt class="constant">SIGFPE</tt>,
and set up an appropriate signal handler.
</dl>
<P>
<dl><dt><b><a name="l2h-330"><tt class="function">turnoff_sigfpe</tt></a></b>()
<dd>
Reset default handling of floating point exceptions.
</dl>
<P>
<dl><dt><b><span class="typelabel">exception</span> <a name="l2h-331"><tt class="exception">FloatingPointError</tt></a></b>
<dd>
After <tt class="function">turnon_sigfpe()</tt> 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.
</dl>
<P>
<p><hr>
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<span class="release-info">Release 2.2, documentation updated on December 21, 2001.</span>
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