Question

I am trying to write a dictionary comprehension.

I have a dictionary like this one:

main_dict = {
    'A' : {'key1' : 'valueA1', 'key2' : 'valueA2'},
    'B' : {'key2' : 'valueB2', 'key3' : 'valueB3'},
    'C' : {'key3' : 'valueC3', 'key1' : 'valueC1'}}

I want to perform the following logic:

d = {}
for k_outer, v_outer in main_dict.items():
    for k_inner, v_inner in v_outer.items():
        if k_inner in d.keys():
            d[k_inner].append([k_outer, v_inner])
        else:
            d[k_inner] = [[k_outer, v_inner]]

Which yields the following result:

{'key3': [['C', 'valueC3'], ['B', 'valueB3']], 
 'key2': [['A', 'valueA2'], ['B', 'valueB2']], 
 'key1': [['A', 'valueA1'], ['C', 'valueC1']]}

(I know I could use defaultdict(list) but this is just an example)

I want to perform the logic using a dict-comprehension, so far I have the following:

d = {k : [m, v] for m, x in main_dict.items() for k, v in x.items()}

This does not work, it only gives me the following output:

{'key3' : ['B', 'valueB3'],
'key2' : ['B', 'valueB2'],
'key1' : ['C', 'valueC1']}

Which is the last instance found for each inner_key…

I am at a loss of how to perform this nested list-comprehension correctly. I have tried multiple variations, all worse than the last.

Answer 1

in the end, this is what i used:

from collections import defaultdict

d = defaultdict(list)
for m, x in main_dict.items():
    for k, v in x.items():
        d[k].append((m, v))