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From: Russell Owen <rowen@uw.edu>
Subject: Re: Best way to calculate fraction part of x?
Date: Thu, 26 Mar 2015 15:02:56 -0700
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Xref: csiph.com comp.lang.python:88100

On 3/24/15 6:39 PM, Jason Swails wrote:
>
>
> On Mon, Mar 23, 2015 at 8:38 PM, Emile van Sebille <emile@fenx.com
> <mailto:emile@fenx.com>> wrote:
>
>     On 3/23/2015 5:52 AM, Steven D'Aprano wrote:
>
>         Are there any other, possibly better, ways to calculate the
>         fractional part
>         of a number?
>
>
>     float (("%6.3f" % x)[-4:])
>
>
> ​In general you lose a lot of precision this way...​

I suggest modf in the math library:

math.modf(x)
Return the fractional and integer parts of x. Both results carry the 
sign of x and are floats.