Good coverage. I use it quite often for cache dictionary, it's much simpler API and overall code, than creatng a new class for it. Demo snippet from effbot's site:

  def calculate(a, b, c, memo={}):
    try:
      value = memo[a, b, c] # return already calculated value
    except KeyError:
      value = heavy_calculation(a, b, c)
      memo[a, b, c] = value # update the memo dictionary
      return value

This seems a bit leaky. You're exposing the caching mechanism in the method signature(yeah, ok, in practice it's unlikely to be a problem).

The other option is to create your own cache object and pass that in (and around, if it's a recursive function). Of course, in Python pretty much every cache object follows the dictionary interface anyway, so it doesn't really matter. One of the benefits of duck typing :)