Casting large int value to float with no round up

Question

Today I found out that casting int to float have pretty interesting precision and rounding rules. I have couple of numbers which gives me unexpected results:

float(1641016854000000000) -> 1.641016854e+18 (as expected)
float(1641016854000000100) -> 1.641016854e+18 (expected 1.6410168540000001e18)
float(1641016854000000200) -> 1.6410168540000003e+18 (expected 1.6410168540000002e+18)
float(1641016854000000300) -> 1.6410168540000003e+18 (as expected)
float(1641016854000000400) -> 1.6410168540000005e+18 (expected 1.6410168540000004e+18)

So my questions are:

1. How I can prevent rounding of the values?
2. How to get expected results instead those I have now?

Answer 1

Python's default float type, as with most languages, can only be so precise to conserve memory. This is why you get floating point errors when working with extremely precise numbers. A good way to avoid this is to use python's decimal library.