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| 54 | ||
| 55 | <H1><A NAME="SECTION005400000000000000000"> | |
| 56 | 3.4 <tt class="module">fpectl</tt> -- | |
| 57 | Floating point exception control</A> | |
| 58 | </H1> | |
| 59 | ||
| 60 | <P> | |
| 61 | <A NAME="module-fpectl"></A> | |
| 62 | <p class="availability">Availability: <span | |
| 63 | class="platform">Unix</span>.</p> | |
| 64 | ||
| 65 | <P> | |
| 66 | Most computers carry out floating point operations<a id='l2h-434' xml:id='l2h-434'></a> | |
| 67 | in conformance with the so-called IEEE-754 standard. | |
| 68 | On any real computer, | |
| 69 | some floating point operations produce results that cannot | |
| 70 | be expressed as a normal floating point value. | |
| 71 | For example, try | |
| 72 | ||
| 73 | <P> | |
| 74 | <div class="verbatim"><pre> | |
| 75 | >>> import math | |
| 76 | >>> math.exp(1000) | |
| 77 | inf | |
| 78 | >>> math.exp(1000) / math.exp(1000) | |
| 79 | nan | |
| 80 | </pre></div> | |
| 81 | ||
| 82 | <P> | |
| 83 | (The example above will work on many platforms. | |
| 84 | DEC Alpha may be one exception.) | |
| 85 | "Inf" is a special, non-numeric value in IEEE-754 that | |
| 86 | stands for "infinity", and "nan" means "not a number." | |
| 87 | Note that, | |
| 88 | other than the non-numeric results, | |
| 89 | nothing special happened when you asked Python | |
| 90 | to carry out those calculations. | |
| 91 | That is in fact the default behaviour prescribed in the IEEE-754 standard, | |
| 92 | and if it works for you, | |
| 93 | stop reading now. | |
| 94 | ||
| 95 | <P> | |
| 96 | In some circumstances, | |
| 97 | it would be better to raise an exception and stop processing | |
| 98 | at the point where the faulty operation was attempted. | |
| 99 | The <tt class="module">fpectl</tt> module | |
| 100 | is for use in that situation. | |
| 101 | It provides control over floating point | |
| 102 | units from several hardware manufacturers, | |
| 103 | allowing the user to turn on the generation | |
| 104 | of <tt class="constant">SIGFPE</tt> whenever any of the | |
| 105 | IEEE-754 exceptions Division by Zero, Overflow, or | |
| 106 | Invalid Operation occurs. | |
| 107 | In tandem with a pair of wrapper macros that are inserted | |
| 108 | into the C code comprising your python system, | |
| 109 | <tt class="constant">SIGFPE</tt> is trapped and converted into the Python | |
| 110 | <tt class="exception">FloatingPointError</tt> exception. | |
| 111 | ||
| 112 | <P> | |
| 113 | The <tt class="module">fpectl</tt> module defines the following functions and | |
| 114 | may raise the given exception: | |
| 115 | ||
| 116 | <P> | |
| 117 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
| 118 | <td><nobr><b><tt id='l2h-431' xml:id='l2h-431' class="function">turnon_sigfpe</tt></b>(</nobr></td> | |
| 119 | <td><var></var>)</td></tr></table></dt> | |
| 120 | <dd> | |
| 121 | Turn on the generation of <tt class="constant">SIGFPE</tt>, | |
| 122 | and set up an appropriate signal handler. | |
| 123 | </dl> | |
| 124 | ||
| 125 | <P> | |
| 126 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
| 127 | <td><nobr><b><tt id='l2h-432' xml:id='l2h-432' class="function">turnoff_sigfpe</tt></b>(</nobr></td> | |
| 128 | <td><var></var>)</td></tr></table></dt> | |
| 129 | <dd> | |
| 130 | Reset default handling of floating point exceptions. | |
| 131 | </dl> | |
| 132 | ||
| 133 | <P> | |
| 134 | <dl><dt><b><span class="typelabel">exception</span> <tt id='l2h-433' xml:id='l2h-433' class="exception">FloatingPointError</tt></b></dt> | |
| 135 | <dd> | |
| 136 | After <tt class="function">turnon_sigfpe()</tt> has been executed, | |
| 137 | a floating point operation that raises one of the | |
| 138 | IEEE-754 exceptions | |
| 139 | Division by Zero, Overflow, or Invalid operation | |
| 140 | will in turn raise this standard Python exception. | |
| 141 | </dd></dl> | |
| 142 | ||
| 143 | <P> | |
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| 151 | <LI><A href="fpectl-example.html">3.4.1 Example</a> | |
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| 191 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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