I spent some time optimising Python's "fractions" module. Fractions (i.e. rational numbers) are great for all sorts of exact computations, especially money calculations. You never have to care about loss of precision, you can freely mix very large and very small numbers any way you like in your computations - the result is always exact as it's all done in integers internally.
But the performance used to suck. Totally. The main problem was the type instantiation, which is really expensive. For example, simply changing this code
(which avoids intermediate Fraction operations) could speed it up by factors. I provided some patches that streamline common cases (numerator and denominator will usually be Python ints), and this made the implementation in CPython 3.5 twice as fast as before. It actually starts being usable. :)
For those who can't wait for Python 3.5 to come out (in about a year's time), and also for those who want even better performance (like me), I dropped the implementation into Cython and optimised it further at the C level. That gave me another factor of 5, so the result is currently about 10x faster than what's in the standard library.
Compared to the Decimal type in Python 2.7, it's about 15x faster. The hugely improved C reimplementation of the "decimal" module in Python 3.3 is still about 5-6x faster - or less, if you often need to rescale your values along the way. Plus, with decimal, you always have to take care of using the right precision scale for your code to prevent rounding errors, and playing it safe will slow it down.
I released the module to PyPI, it's called quicktions. Hope you like it.