You get the same behaviour with regular floats: >>> a=10.0**10 >>> a 10000000000.0 >>> b = 0.1 >>> b+a 10000000000.1 >>> c=b+a

You get the same behaviour with regular floats: >>> a=10.0**10 >>> a 10000000000.0 >>> b = 0.1 >>> b+a 10000000000.1 >>> c=b+a >>> d=c-a >>> d 0.10000038146972656 >>> d == b False >>> a + b - b == b False >>> You could build a variable-tracking system on top of the numbers, or you could just use a symbolic computation package, then simplify the final equations so each variable figures only once.