[mturmon]

I use defaultdict a lot - for accumulators when you're not sure about what is coming. Here's a simplified example:

    # a[star_name][instrument] = set of (seed, planet index) of visited planets
    a = defaultdict(lambda: defaultdict(set))
    for row in rows:
        a[row.star][row.inst].add((row.seed, row.planet))

This is a dict-of-dict-of-set that is accumulating from a stream of rows, and I don't know what stars and instruments will be present.

Another related tool is Counter (https://docs.python.org/3/library/collections.html#collectio...)