[SOLVED] defaultdict of defaultdict?

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()

Solution

Yes like this:

defaultdict(lambda: defaultdict(int))

The argument of a defaultdict (in this case is lambda: defaultdict(int)) will be called when you try to access a key that doesn't exist. The return value of it will be set as the new value of this key, which means in our case the value of d[Key_doesnt_exist] will be defaultdict(int).

If you try to access a key from this last defaultdict i.e. d[Key_doesnt_exist][Key_doesnt_exist] it will return 0, which is the return value of the argument of the last defaultdict i.e. int().