For rational numbers, Python has a Fraction class in the standard library that performs exact integer arithmetic:

    >>> from fractions import Fraction
    >>> f = Fraction(0xFEDCBA987654321, 0x123456789ABCDEF)
    >>> f%1
    Fraction(1, 5465701947765793)
    >>> f - f%1
    Fraction(14, 1)

That shows that 0xFEDCBA987654321 / 0x123456789ABCDEF = 14 + 1/5465701947765793 exactly.

    >>> math.log(5465701947765793, 2)
    52.279328213174445

Shows that the denominator requires 52 bits which is slightly more than the number of mantissa bits in a 64-bit floating point number, so the result gets rounded to 14.0 due to limited precision.