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Clarify that decimal also supports fixed-point arithmetic.

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Raymond Hettinger 2008-02-10 07:21:09 +00:00
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Doc/library/decimal.rst
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@ -1,6 +1,6 @@
:mod:`decimal` --- Decimal floating point arithmetic :mod:`decimal` --- Decimal fixed point and floating point arithmetic
==================================================== ====================================================================
.. module:: decimal .. module:: decimal
:synopsis: Implementation of the General Decimal Arithmetic Specification. :synopsis: Implementation of the General Decimal Arithmetic Specification.
@ -21,6 +21,11 @@
The :mod:`decimal` module provides support for decimal floating point The :mod:`decimal` module provides support for decimal floating point
arithmetic. It offers several advantages over the :class:`float` datatype: arithmetic. It offers several advantages over the :class:`float` datatype:
* Decimal "is based on a floating-point model which was designed with people
in mind, and necessarily has a paramount guiding principle -- computers must
provide an arithmetic that works in the same way as the arithmetic that
people learn at school." -- excerpt from the decimal arithmetic specification.
* Decimal numbers can be represented exactly. In contrast, numbers like * Decimal numbers can be represented exactly. In contrast, numbers like
:const:`1.1` do not have an exact representation in binary floating point. End :const:`1.1` do not have an exact representation in binary floating point. End
users typically would not expect :const:`1.1` to display as users typically would not expect :const:`1.1` to display as
@ -30,7 +35,7 @@ arithmetic. It offers several advantages over the :class:`float` datatype:
+ 0.1 + 0.1 - 0.3`` is exactly equal to zero. In binary floating point, the result + 0.1 + 0.1 - 0.3`` is exactly equal to zero. In binary floating point, the result
is :const:`5.5511151231257827e-017`. While near to zero, the differences is :const:`5.5511151231257827e-017`. While near to zero, the differences
prevent reliable equality testing and differences can accumulate. For this prevent reliable equality testing and differences can accumulate. For this
reason, decimal would be preferred in accounting applications which have strict reason, decimal is preferred in accounting applications which have strict
equality invariants. equality invariants.
* The decimal module incorporates a notion of significant places so that ``1.30 * The decimal module incorporates a notion of significant places so that ``1.30
@ -55,6 +60,13 @@ arithmetic. It offers several advantages over the :class:`float` datatype:
standards. While the built-in float type exposes only a modest portion of its standards. While the built-in float type exposes only a modest portion of its
capabilities, the decimal module exposes all required parts of the standard. capabilities, the decimal module exposes all required parts of the standard.
When needed, the programmer has full control over rounding and signal handling. When needed, the programmer has full control over rounding and signal handling.
This includes an option to enforce exact arithmetic by using exceptions
to block any inexact operations.
* The decimal module was designed to support "without prejudice, both exact
unrounded decimal arithmetic (sometimes called fixed-point arithmetic)
and rounded floating-point arithmetic." -- excerpt from the decimal
arithmetic specification.
The module design is centered around three concepts: the decimal number, the The module design is centered around three concepts: the decimal number, the
context for arithmetic, and signals. context for arithmetic, and signals.