Creating a nested dictionary from a flattened dictionary

Question

I have a flattened dictionary which I want to make into a nested one, of the form

flat = {\'X_a_one\': 10,
        \'X_a_two\': 20, 
        \'X_b_one\': 10,         


        

Answer 1

Here's one way using collections.defaultdict, borrowing heavily from this previous answer. There are 3 steps:

1. Create a nested defaultdict of defaultdict objects.
2. Iterate items in flat input dictionary.
3. Build defaultdict result according to the structure derived from splitting keys by _, using getFromDict to iterate the result dictionary.

This is a complete example:

from collections import defaultdict
from functools import reduce
from operator import getitem

def getFromDict(dataDict, mapList):
    """Iterate nested dictionary"""
    return reduce(getitem, mapList, dataDict)

# instantiate nested defaultdict of defaultdicts
tree = lambda: defaultdict(tree)
d = tree()

# iterate input dictionary
for k, v in flat.items():
    *keys, final_key = k.split('_')
    getFromDict(d, keys)[final_key] = v

{'X': {'a': {'one': 10, 'two': 20}, 'b': {'one': 10, 'two': 20}},
 'Y': {'a': {'one': 10, 'two': 20}, 'b': {'one': 10, 'two': 20}}}

As a final step, you can convert your defaultdict to a regular dict, though usually this step is not necessary.

def default_to_regular_dict(d):
    """Convert nested defaultdict to regular dict of dicts."""
    if isinstance(d, defaultdict):
        d = {k: default_to_regular_dict(v) for k, v in d.items()}
    return d

# convert back to regular dict
res = default_to_regular_dict(d)