Published: October 2010
By Jessie Anderson
In Code .

tags: python modules math

The built in math functions in Python use binary approximations, giving some funky results when dealing with numbers containing decimals:

>>> . 1 + . 2
0.30000000000000004
>>> round ( 100.00 / 3.000 , 4 )
33.333300000000001
One way to appropriately find the sum of decimals is to use strings

Also, the default is to round to the nearest whole number when dividing

>>> 1 / 3
0
>>> 100 / 3
33
The decimal library is a useful tool for floating point arithmetic. Instead of the command 'from decimal import * ' that would import everything from decimal, all I need to import is Decimal and getcontext. When importing modules, simplicity is preferred. There are less problems with naming in your code and you can be more aware of the tools at your disposal. I already imported decimal and looked through the directory to determine which modules I wanted. I'm only going to show the precision feature of decimal. You may want to import the entire library if you want to use other functions.

>>> from decimal import getcontext
>>> from decimal import Decimal
>>> getcontext ()
Context ( prec = 28 , rounding = ROUND_HALF_UP , Emin =- 999999999 ,
Emax = 999999999 , capitals = 1 , flags = [ Inexact , Rounded ],
traps = [ DivisionByZero , Overflow , InvalidOperation ])
>>> #our precision is also known as significant figures, applied after arithmetic
... #let's change our precision
...
>>> getcontext () . prec = 6
>>> Decimal ( '1' ) / Decimal ( '7' ) # can be performed to strings
Decimal ( '0.142857' )
>>> Decimal ( 1 ) / Decimal ( 7 ) # can be performed to integers
Decimal ( '0.142857' )
>>> Decimal ( 10 ) / Decimal ( 7 )
Decimal ( '1.42857' ) # notice that 6 is the total number of figures, not the number after the decimal
>>> Decimal ( 10 ) / Decimal ( 5 )
Decimal ( '2' ) # not '2.00000,' which is considered more accurate than 2 by the science community
As someone with a science background, I found the decimal library's use of 'significant figures' interesting. Decimal can also be used in financial reporting or billing. You can also find maximums and minimums, change rounding properties, and do anything that you can do with the math library. I personally prefer the math library for the algebraic functions performed by decimal, because math's syntax is simpler. To learn more about decimal, go to http://docs.python.org/library/decimal.html