Reverse dictionary having list:key into key:list

Question

I have dictionaries like :

Letters_value = dict.fromkeys(['a', 'b', 'c'], 2)
Letters_value.update(dict.fromkeys(['d','e', 'f'], 3))
Letters_value.update(dict.fromkeys(['g','h', 'i'], 4))
Letters_value.update(dict.fromkeys(['j','k', 'l'], 5))
Letters_value.update(dict.fromkeys(['m','n', 'o'], 6))
Letters_value.update(dict.fromkeys(['p','q', 'r','s'], 7))
Letters_value.update(dict.fromkeys(['t','u', 'v'],8 ))
Letters_value.update(dict.fromkeys(['w','x', 'y','z'],9 ))
Letters_value.update(dict.fromkeys(['-'],'' ))

Is there a nice way to reverse them so that I get something like :

{'2':['a','b','c'], '3':['d','e','f'].... }

Possible duplicate of [Python reverse / invert a mapping](https://stackoverflow.com/questions/483666/python-reverse-invert-a-mapping) — jonrsharpe, Sep 05 '18 at 07:22
if you can use `import pandas as pd` then you can do :`pd.DataFrame(Letters_value,[0]).T.reset_index().groupby(0).agg(list).to_dict().get('index')` — Onyambu, Sep 05 '18 at 07:43
Not t a duplicate , please try to reverse the dict that I gave . — Remi-007, Sep 05 '18 at 07:49

score 1 · Accepted Answer · answered Sep 05 '18 at 07:20

Using collections.defaultdict

Ex:

from collections import defaultdict


Letters_value = dict.fromkeys(['a', 'b', 'c'], 2)
Letters_value.update(dict.fromkeys(['d','e', 'f'], 3))
Letters_value.update(dict.fromkeys(['g','h', 'i'], 4))
Letters_value.update(dict.fromkeys(['j','k', 'l'], 5))
Letters_value.update(dict.fromkeys(['m','n', 'o'], 6))
Letters_value.update(dict.fromkeys(['p','q', 'r','s'], 7))
Letters_value.update(dict.fromkeys(['t','u', 'v'],8 ))
Letters_value.update(dict.fromkeys(['w','x', 'y','z'],9 ))
Letters_value.update(dict.fromkeys(['-'],'' ))

res = defaultdict(list)
for k, v in Letters_value.items():
    res[v].append(k)
print(res)

Output:

defaultdict(<type 'list'>, {'': ['-'], 2: ['a', 'c', 'b'], 3: ['e', 'd', 'f'], 4: ['g', 'i', 'h'], 5: ['k', 'j', 'l'], 6: ['m', 'o', 'n'], 7: ['q', 'p', 's', 'r'], 8: ['u', 't', 'v'], 9: ['w', 'y', 'x', 'z']})

Thank you it is exactly what I was needed , I didn't about from collections import defaultdict. Have a nice day :) — Remi-007, Sep 05 '18 at 07:46

score -1 · Answer 2 · answered Sep 05 '18 at 07:33

-1

We can swap the positions of key-value pair in a dictionary by the following line:

dict((v, k) for k, v in Letters_value.items())

or you can also use,

{v: k for k, v in Letters_value.items()}

I hope it can help you. :-)

answered Sep 05 '18 at 07:33

Sukjun Kim

187
1
5

This does not work since most of the information would be lost (there are multiple identical values in the original dictionary, but keys need to be unique). – Tim Pietzcker Sep 05 '18 at 07:42
@TimPietzcker Yes, I know. I have only focused on the methodology for reversing/inverting a mapping of a dictionary data. We have to check whether original values in a dictionary are unique of values of a dictionary prior to reversing/inverting a mapping. I answered it with the assumption that the example above(`Letters_value`) already meets the condition of uniqueness. – Sukjun Kim Sep 05 '18 at 07:56
@TimPietzcker Thank you for letting me know. I missed the point of the question. I agree with your comment. It became a chance to think about the loss of information. :) – Sukjun Kim Sep 05 '18 at 15:27

Reverse dictionary having list:key into key:list

2 Answers2