Question

How do I count the number of subelements in a nested dictionary in the most efficient manner possible? The len() function doesn’t work as I initially expected it to:

>>> food_colors = {'fruit': {'orange': 'orange', 'apple': 'red', 'banana': 'yellow'}, 'vegetables': {'lettuce': 'green', 'beet': 'red', 'pumpkin': 'orange'}}
>>> len(food_colors)
2
>>>

What if I actually want to count the number of subelements? (e.g., expected result to be “6”) Is there a better way to do this rather than looping through each element and summing the qty of subelements? In this particular application, I have about five million subelements to count and every clock cycle counts.

Answer 1

Is it guaranteed that each top-level key has a dictionary as its value, and that no second-level key has a dictionary? If so, this will go as fast as you can hope for:

sum(len(v) for v in food_colors.itervalues())

If the data structure is more complicated, it will need more code, of course. I’m not aware of any intrinsics to do deep data structure walks.