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An Introduction to Python
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 124 pages
ISBN 0954161769
RRP £12.95 ($19.95)

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11.8 Decimal Floating Point Arithmetic

The ‘decimal’ module offers a Decimal datatype for decimal floating point arithmetic. Compared to the built-in float implementation of binary floating point, the new class is especially helpful 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 calculations done by hand.

For example, calculating a 5% tax on a 70 cent phone charge gives different results in decimal floating point and binary floating point. The difference becomes significant if the results are rounded to the nearest cent:

    >>> from decimal import *       
    >>> Decimal('0.70') * Decimal('1.05')
    Decimal("0.7350")
    >>> .70 * 1.05
    0.73499999999999999       

The Decimal result keeps a trailing zero, automatically inferring four place significance from multiplicands with two place significance. Decimal reproduces mathematics as done by hand and avoids issues that can arise when binary floating point cannot exactly represent decimal quantities.

Exact representation enables the Decimal class to perform modulo calculations and equality tests that are unsuitable for binary floating point:

    >>> 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      

The ‘decimal’ module provides arithmetic with as much precision as needed:

    >>> getcontext().prec = 36
    >>> Decimal(1) / Decimal(7)
    Decimal("0.142857142857142857142857142857142857")
ISBN 0954161769An Introduction to PythonSee the print edition