Python Float rounding errors [duplicate]

Answer 1

List comprehension does not matter.

>>> 3 * 0.1
0.30000000000000004
>>> 2 * 0.1
0.2
>>> 0.1 + 0.2
0.30000000000000004

More information about Python float and floating point arithmetic - here

Answer 2

floats are inherently imprecise in pretty much every language

if you need exact precision use the Decimal class

from decimal import Decimal
print Decimal("0.3")

if you just need them to look pretty just use format strings when displaying eg :

"%0.2f"%2.030000000000034

if you want to compare them use some threshold

if num1 - num2 < 1e-3 : print "Equal Enough For Me!"

**see abarnert's comments on thresholding ... this is a very simplified example for a more indepth explanation of epsilon thresholding one article I found is here http://www.cygnus-software.com/papers/comparingfloats/Comparing%20floating%20point%20numbers.htm

Additional Reading:

http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html (for a detailed explanation)

http://floating-point-gui.de/basic/ (basic tutorial for working with floats in general)

Answer 3

The list comprehension is irrelevant, this is purely an issue with floating-point numbers. For an extremely detailed answer you should give this article a read: http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html