Question

After reading What is the best way to implement nested dictionaries? why is it wrong to do:

c = collections.defaultdict(collections.defaultdict(int))

in python? I would think this would work to produce

{key:{key:1}}

or am I thinking about it wrong?

Answer 1

The constructor of defaultdict expects a callable. defaultdict(int) is a default dictionary object, not a callable. Using a lambda it can work, however:

c = collections.defaultdict(lambda: collections.defaultdict(int))

This works since what I pass to the outer defaultdict is a callable that creates a new defaultdict when called.

Here’s an example:

>>> import collections
>>> c = collections.defaultdict(lambda: collections.defaultdict(int))
>>> c[5][6] += 1
>>> c[5][6]
1
>>> c[0][0]
0
>>>