A dictionary subclass that deals with key collisions by accumulating the values with a custom operator.

Commenter Nathan added:

For counting things I use the collections.Counter class. In addition to giving you a one-liner for obtaining counts from a sequence or other iterable, the most_common() method makes it easy to retrieve the highest-count item(s).

The reminder of the Counter class was useful, since it solves many of the same problems as AccumulationDict. I then countered (ha!) by noting that Counter only increments for each key collision, whereas AccumulationDict can track counts when you already have some frequency information, or when you want to do more than count things (accumulate items in a list, for instance).