Python - Flooring floats

Question

This is a really simple question. Lets denote the following:

>>> x = 1.2876

Now, round has this great optional second parameter that will round at that decimal place:

>>> round(x,3)
1.288

I was wondering if there is a simple way to round down the numbers. math.floor(x,3) returns an error rather than 1.287

Answer 1

This may be the easiest, if by "rounding down" you mean "toward minus infinity" (as floor() does):

>>> x = 1.2876
>>> x - x % .001
1.287
>>> x = -1.1111
>>> x - x % .001
-1.112

This is prone to lots of shallow surprises, though, due to that most decimal values cannot be exactly represented as binary floating-point values. If those bother you, do something similar with decimal.Decimal values instead.