Switching key:value list to have values as keys?

morais12 :

I have a CSV where the data is structured like so:

Key | Value
A | 1, 2, 3
B | 4, 5
C | 1, 3, 5

I want to convert it into a CSV that looks like this:

Key | Value
1 | A, C
2 | A
3 | A, C
4 | B
5 | B, C

The current values are strings (not numbers) separated by commas. The keys are not unique - a key can appear multiple times, with different values. I've only done this previously where there was one value per line, not multiple.

Óscar López :

Assuming that the input has been loaded into a dictionary of lists, this should work for making the conversion:

from collections import defaultdict

input_dict = { 'A': [1, 2, 3], 'B': [4, 5], 'C': [1, 3, 5] }
output_dict = defaultdict(list)

for k, v in input_dict.items():
    for e in v:
        output_dict[e].append(k)

The result is as expected:

output_dict
=> defaultdict(<type 'list'>, {1: ['A', 'C'], 2: ['A'], 3: ['A', 'C'], 4: ['B'], 5: ['C', 'B']})

Now you can write the output to a CSV file with the format you want.

Switching key:value list to have values as keys?

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