pytoolz/cytoolz — pull request #40 by eriknw (2014-06-28 22:58:31+00:00)
Title: groupby can accept an index or list of indices as the key

The approach taken here is to move the core of the calculation to inline function `_groupby_core`. This reduces the amount of repeated code while maintaining very high performance. `reduceby` and `countby` will use a similar structure of branching and iteration.

In case there was any doubt, performing this optimization (instead of using a "getter" function) is most definitely worth it!