How to implement an ordered, default dict? [duplicate]

Answer 1

If you want a simple solution that doesn't require a class, you can just use OrderedDict.setdefault(key, default=None) or OrderedDict.get(key, default=None). If you only get / set from a few places, say in a loop, you can easily just setdefault.

totals = collections.OrderedDict()

for i, x in some_generator():
    totals[i] = totals.get(i, 0) + x

It is even easier for lists with setdefault:

agglomerate = collections.OrderedDict()

for i, x in some_generator():
    agglomerate.setdefault(i, []).append(x)

But if you use it more than a few times, it is probably better to set up a class, like in the other answers.