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()
I find it slightly more elegant to use partial:
import functools
dd_int = functools.partial(defaultdict, int)
defaultdict(dd_int)
Of course, this is the same as a lambda.
Others have answered correctly your question of how to get the following to work:
for x in stuff:
d[x.a][x.b] += x.c_int
An alternative would be to use tuples for keys:
d = defaultdict(int)
for x in stuff:
d[x.a,x.b] += x.c_int
# ^^^^^^^ tuple key
The nice thing about this approach is that it is simple and can be easily expanded. If you need a mapping three levels deep, just use a three item tuple for the key.