[gamegoblin]

I use defaultdict all the time. Very often when describing graphs.

  g = defaultdict(dict)

Allows you to do

  g[node1][node2] = edge_weight

without checking if node1 exists, and if not, saying g[node1] = {}

Also a neat trick (of dubious use) is:

  def auto_tree(): return defaultdict(auto_tree)

Gives you infinitely nested defaultdicts.

[hcarvalhoalves]

defaultdict is really multipurpose, quick trees is only one of the neat tricks. I like the following definition :)

    >>> Tree = lambda: defaultdict(Tree)