Multiple levels of 'collection.defaultdict' in Python

Thanks to some great folks on SO, I discovered the possibilities offered by collections.defaultdict, notably in readability and speed. I have put them to use with success.

Now I would like to implement three levels of dictionaries, the two top ones being defaultdict and the lowest one being int. I don't find the appropriate way to do this. Here is my attempt:

from collections import defaultdict
d = defaultdict(defaultdict)
a = [("key1", {"a1":22, "a2":33}),
     ("key2", {"a1":32, "a2":55}),
     ("key3", {"a1":43, "a2":44})]
for i in a:
    d[i[0]] = i[1]

Now this works, but the following, which is the desired behavior, doesn't:

d["key4"]["a1"] + 1

I suspect that I should have declared somewhere that the second level defaultdict is of type int, but I didn't find where or how to do so.

The reason I am using defaultdict in the first place is to avoid having to initialize the dictionary for each new key.

Any more elegant suggestion?

Thanks pythoneers!

Solution 1:

Use:

from collections import defaultdict
d = defaultdict(lambda: defaultdict(int))

This will create a new defaultdict(int) whenever a new key is accessed in d.

Solution 2:

Another way to make a pickleable, nested defaultdict is to use a partial object instead of a lambda:

from functools import partial
...
d = defaultdict(partial(defaultdict, int))

This will work because the defaultdict class is globally accessible at the module level:

"You can't pickle a partial object unless the function [or in this case, class] it wraps is globally accessible ... under its __name__ (within its __module__)" -- Pickling wrapped partial functions