FWIW, here is my take on the defaultdict approach:
def defaultdict(defaultfactory, dictclass=dict):
class defdict(dictclass):
def __getitem__(self, key):
try:
return super(defdict, self).__getitem__(key)
except KeyError:
return self.setdefault(key, defaultfactory())
return defdict
d = defaultdict(int)()
d["x"] += 1
d["x"] += 1
d["y"] += 1
print d
d = defaultdict(list)()
d["x"].append(1)
d["x"].append(2)
d["y"].append(1)
print d
Michele Simionato
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