Creating a nested dictionary from a flattened dictionary

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

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