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<title>5.6.4 Signals </title>
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<H2><A NAME="SECTION007640000000000000000"></A><A NAME="decimal-signals"></A>
<BR>
5.6.4 Signals
</H2>
<P>
Signals represent conditions that arise during computation.
Each corresponds to one context flag and one context trap enabler.
<P>
The context flag is incremented whenever the condition is encountered.
After the computation, flags may be checked for informational
purposes (for instance, to determine whether a computation was exact).
After checking the flags, be sure to clear all flags before starting
the next computation.
<P>
If the context's trap enabler is set for the signal, then the condition
causes a Python exception to be raised. For example, if the
<tt class="class">DivisionByZero</tt> trap is set, then a <tt class="exception">DivisionByZero</tt>
exception is raised upon encountering the condition.
<P>
<dl><dt><b><span class="typelabel">class</span> <tt id='l2h-1240' xml:id='l2h-1240' class="class">Clamped</tt></b>
<dd>
Altered an exponent to fit representation constraints.
<P>
Typically, clamping occurs when an exponent falls outside the context's
<tt class="member">Emin</tt> and <tt class="member">Emax</tt> limits. If possible, the exponent is
reduced to fit by adding zeroes to the coefficient.
</dl>
<P>
<dl><dt><b><span class="typelabel">class</span> <tt id='l2h-1241' xml:id='l2h-1241' class="class">DecimalException</tt></b>
<dd>
Base class for other signals and is a subclass of
<tt class="exception">ArithmeticError</tt>.
</dl>
<P>
<dl><dt><b><span class="typelabel">class</span> <tt id='l2h-1242' xml:id='l2h-1242' class="class">DivisionByZero</tt></b>
<dd>
Signals the division of a non-infinite number by zero.
<P>
Can occur with division, modulo division, or when raising a number to a
negative power. If this signal is not trapped, returns
<tt class="constant">Infinity</tt> or <tt class="constant">-Infinity</tt> with the sign determined by
the inputs to the calculation.
</dl>
<P>
<dl><dt><b><span class="typelabel">class</span> <tt id='l2h-1243' xml:id='l2h-1243' class="class">Inexact</tt></b>
<dd>
Indicates that rounding occurred and the result is not exact.
<P>
Signals when non-zero digits were discarded during rounding. The rounded
result is returned. The signal flag or trap is used to detect when
results are inexact.
</dl>
<P>
<dl><dt><b><span class="typelabel">class</span> <tt id='l2h-1244' xml:id='l2h-1244' class="class">InvalidOperation</tt></b>
<dd>
An invalid operation was performed.
<P>
Indicates that an operation was requested that does not make sense.
If not trapped, returns <tt class="constant">NaN</tt>. Possible causes include:
<P>
<div class="verbatim"><pre>
Infinity - Infinity
0 * Infinity
Infinity / Infinity
x % 0
Infinity % x
x._rescale( non-integer )
sqrt(-x) and x > 0
0 ** 0
x ** (non-integer)
x ** Infinity
</pre></div>
</dl>
<P>
<dl><dt><b><span class="typelabel">class</span> <tt id='l2h-1245' xml:id='l2h-1245' class="class">Overflow</tt></b>
<dd>
Numerical overflow.
<P>
Indicates the exponent is larger than <tt class="member">Emax</tt> after rounding has
occurred. If not trapped, the result depends on the rounding mode, either
pulling inward to the largest representable finite number or rounding
outward to <tt class="constant">Infinity</tt>. In either case, <tt class="class">Inexact</tt> and
<tt class="class">Rounded</tt> are also signaled.
</dl>
<P>
<dl><dt><b><span class="typelabel">class</span> <tt id='l2h-1246' xml:id='l2h-1246' class="class">Rounded</tt></b>
<dd>
Rounding occurred though possibly no information was lost.
<P>
Signaled whenever rounding discards digits; even if those digits are
zero (such as rounding <tt class="constant">5.00</tt> to <tt class="constant">5.0</tt>). If not
trapped, returns the result unchanged. This signal is used to detect
loss of significant digits.
</dl>
<P>
<dl><dt><b><span class="typelabel">class</span> <tt id='l2h-1247' xml:id='l2h-1247' class="class">Subnormal</tt></b>
<dd>
Exponent was lower than <tt class="member">Emin</tt> prior to rounding.
<P>
Occurs when an operation result is subnormal (the exponent is too small).
If not trapped, returns the result unchanged.
</dl>
<P>
<dl><dt><b><span class="typelabel">class</span> <tt id='l2h-1248' xml:id='l2h-1248' class="class">Underflow</tt></b>
<dd>
Numerical underflow with result rounded to zero.
<P>
Occurs when a subnormal result is pushed to zero by rounding.
<tt class="class">Inexact</tt> and <tt class="class">Subnormal</tt> are also signaled.
</dl>
<P>
The following table summarizes the hierarchy of signals:
<P>
<div class="verbatim"><pre>
exceptions.ArithmeticError(exceptions.StandardError)
DecimalException
Clamped
DivisionByZero(DecimalException, exceptions.ZeroDivisionError)
Inexact
Overflow(Inexact, Rounded)
Underflow(Inexact, Rounded, Subnormal)
InvalidOperation
Rounded
Subnormal
</pre></div>
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<span class="release-info">Release 2.4.3, documentation updated on 29 March 2006.</span>
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