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Diffstat (limited to 'Lib/numbers.py')
| -rw-r--r-- | Lib/numbers.py | 26 |
1 files changed, 26 insertions, 0 deletions
diff --git a/Lib/numbers.py b/Lib/numbers.py index b5150d22ff..a4b7a6d260 100644 --- a/Lib/numbers.py +++ b/Lib/numbers.py @@ -46,6 +46,32 @@ Inexact.register(float) # Inexact.register(decimal.Decimal) +## Notes on Decimal and it how relates to the numeric tower +## -------------------------------------------------------- +## Decimal is Real except that it does not support rich comparisons. +## +## Decimal has some of the characteristics of Integrals. It provides +## logical operations but not as operators. The logical operations only apply +## to a subset of decimals (those that are non-negative, have a zero exponent, +## and have digits that are only 0 or 1). It does provide __long__() and +## a three argument form of __pow__ that includes exactness guarantees. +## It does not provide an __index__() method. +## +## Depending on context, decimal operations may be exact or inexact. +## +## When decimal is run in a context with small precision and automatic rounding, +## it is Inexact. See the "Floating point notes" section of the decimal docs +## for an example of losing the associative and distributive properties of +## addition. +## +## When decimal is used for high precision integer arithmetic, it is Exact. +## When the decimal used as fixed-point, it is Exact. +## When it is run with sufficient precision, it is Exact. +## When the decimal.Inexact trap is set, decimal operations are Exact. +## For an example, see the float_to_decimal() recipe in the "Decimal FAQ" +## section of the docs -- it shows an how traps are used in conjunction +## with variable precision to reliably achieve exact results. + class Complex(Number): """Complex defines the operations that work on the builtin complex type. |