Move Decimal from the sandbox into production.

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Raymond Hettinger 2004-07-01 11:52:15 +00:00
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\end{seealso}
%======================================================================
\section{PEP 327: Decimal Data Type}
A new module, \module{decimal}, offers a \class{Decimal} data type for
decimal floating point arithmetic. Compared to the built-in \class{float}
type implemented with binary floating point, the new class is especially
useful for financial applications and other uses which require exact
decimal representation, control over precision, control over rounding
to meet legal or regulatory requirements, tracking of significant
decimal places, or for applications where the user expects the results
to match hand calculations done the way they were taught in school.
For example, calculating a 5% tax on a 70 cent phone charge gives
different results in decimal floating point and binary floating point
with the difference being significant when rounding to the nearest
cent:
\begin{verbatim}
>>> from decimal import *
>>> Decimal('0.70') * Decimal('1.05')
Decimal("0.7350")
>>> .70 * 1.05
0.73499999999999999
\end{verbatim}
Note that the \class{Decimal} result keeps a trailing zero, automatically
inferring four place significance from two digit mulitiplicands. A key
goal is to reproduce the mathematics we do by hand and avoid the tricky
issues that arise when decimal numbers cannot be represented exactly in
binary floating point.
Exact representation enables the \class{Decimal} class to perform
modulo calculations and equality tests that would fail in binary
floating point:
\begin{verbatim}
>>> Decimal('1.00') % Decimal('.10')
Decimal("0.00")
>>> 1.00 % 0.10
0.09999999999999995
>>> sum([Decimal('0.1')]*10) == Decimal('1.0')
True
>>> sum([0.1]*10) == 1.0
False
\end{verbatim}
The \module{decimal} module also allows arbitrarily large precisions to be
set for calculation:
\begin{verbatim}
>>> getcontext().prec = 24
>>> Decimal(1) / Decimal(7)
Decimal("0.142857142857142857142857")
\end{verbatim}
\begin{seealso}
\seepep{327}{Decimal Data Type}{Written by Facundo Batista and implemented
by Eric Price, Facundo Bastista, Raymond Hettinger, Aahz, and Tim Peters.}
\end{seealso}
%======================================================================
\section{Other Language Changes}