iterator/generator

[Patrick Maupin]

First of all, as Steve Holden mentioned, do look at xlrd.  It's
awesome.

Second, for your (a) question, if you want an iterator, that's quite
easy:

matriz = iter(matriz)
matriz.next()  # Discard the first one
for i in matriz:

This technique works really well, especially if you have sub-loops.
Then, if you use a defaultdict which uses a list for a constructor

    for i in matriz:
        if i[a] and i[b]:
            g[i[a]].append(i[b])

HTH,
Pat