An Introduction to Pythonby Guido van Rossum and Fred L. Drake, Jr. Paperback (6"x9"), 124 pages ISBN 0954161769 RRP £12.95 ($19.95) Sales of this book support the Python Software Foundation! Get a printed copy>>> |

## 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")

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