Python's decimal module handles fixed-point arithmetic calculations, and before Python 3.3, was implemented in pure Python. The fact that there is a reference implementation written in Python is excellent. Its portable, it means the source is readable for Python people, and it simplifies the tracebacks.

But it's also slow. And this was a problem. I project I was working on recently involved a lot of calculations with Decimal objects, and profiling showed that a huge chunk of time was going on the __mul__ method of Decimal, so I wanted to use cdecimal.Decimal instead of decimal.Decimal. But lots of the values came from the database, and where stored in Django in DecimalFields, which return standard Decimals, and the two types of Decimals are not interoperable. How could I make it so that the DecimalFields gave the cdecimal versions, not the standard ones?

Turns out that because they are drop-in replacements, you can update sys.modules:

import sys
import cdecimal

sys.modules['decimal'] = cdecimal

So I just needed to put this segment at the top of manage.py(for the devserver and shell commands) and wsgi.py (for the WSGI server). And then my view function that was taking more than 30 seconds -- and dying horribly against Heroku's hard 30 second limit -- was now taking less than five seconds.

There were two things that really impressed me about this. Firstly, that cdecimal was so much faster. As of CPython 3.3, it is now the default implementation (and, I read, has had even more performance improvements). Secondly, how well Python accommodates module-level drop in replacements. Guido's time machine strikes again.