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

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.