defaultdict of defaultdict?

Is there a way to have a defaultdict(defaultdict(int)) in order to make the following code work?

for x in stuff:
    d[x.a][x.b] += x.c_int

d needs to be built ad-hoc, depending on x.a and x.b elements.

I could use:

for x in stuff:
    d[x.a,x.b] += x.c_int

but then I wouldn't be able to use:

d.keys()
d[x.a].keys()