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,         'X_b_two': 20,          'Y_a_one': 10,         'Y_a_two': 20,         'Y_b_one': 10,         'Y_b_two': 20} 

I want to convert it to the form

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

The structure of the flat dictionary is such that there should not be any problems with ambiguities. I want it to work for dictionaries of arbitrary depth, but performance is not really an issue. I've seen lots of methods for flattening a nested dictionary, but basically none for nesting a flattened dictionary. The values stored in the dictionary are either scalars or strings, never iterables.

So far I have got something which can take the input

test_dict = {'X_a_one': '10',              'X_b_one': '10',              'X_c_one': '10'} 

to the output

test_out = {'X': {'a_one': '10',                    'b_one': '10',                    'c_one': '10'}} 

using the code

def nest_once(inp_dict):     out = {}     if isinstance(inp_dict, dict):         for key, val in inp_dict.items():             if '_' in key:                 head, tail = key.split('_', 1)                  if head not in out.keys():                     out[head] = {tail: val}                 else:                     out[head].update({tail: val})             else:                 out[key] = val     return out  test_out = nest_once(test_dict) 

But I'm having trouble working out how to make this into something which recursively creates all levels of the dictionary.

Any help would be appreciated!

(As for why I want to do this: I have a file whose structure is equivalent to a nested dict, and I want to store this file's contents in the attributes dictionary of a NetCDF file and retrieve it later. However NetCDF only allows you to put flat dictionaries as the attributes, so I want to unflatten the dictionary I previously stored in the NetCDF file.)

Answer 1

Here is my take:

def nest_dict(flat):     result = {}     for k, v in flat.items():         _nest_dict_rec(k, v, result)     return result  def _nest_dict_rec(k, v, out):     k, *rest = k.split('_', 1)     if rest:         _nest_dict_rec(rest[0], v, out.setdefault(k, {}))     else:         out[k] = v  flat = {'X_a_one': 10,         'X_a_two': 20,          'X_b_one': 10,         'X_b_two': 20,          'Y_a_one': 10,         'Y_a_two': 20,         'Y_b_one': 10,         'Y_b_two': 20} nested = {'X': {'a': {'one': 10,                       'two': 20},                  'b': {'one': 10,                       'two': 20}},            'Y': {'a': {'one': 10,                       'two': 20},                 'b': {'one': 10,                       'two': 20}}} print(nest_dict(flat) == nested) # True