Get key by value in dictionary of lists -python

April 1, 2022

I have a dictionary containing keys that at the same time the key contains a certain list of integers. I want to built a dict that shows me the values as keys and the values of this new dict would be a list of the previous keys that contained the previous value. For example, I have:

turns

{'Monday_2022_03_14': [25, 17, 2, 32, 43, 40, 23, 7, 19, 16],
 'Tuesday_2022_03_15': [33, 10, 16, 31, 25, 22, 43, 41, 21, 28, 3, 30, 29],
 'Wednesday_2022_03_16': [19, 27, 40, 3, 9, 2, 14, 21, 44, 17],
 'Thursday_2022_03_17': [16, 23, 1, 30, 29, 44, 5, 42, 27, 19],
 'Friday_2022_03_18': [6, 17, 2, 29, 27, 41, 44, 5, 40, 42]}

I would like to have, as an example:

{1:['Thursday_2022_03_17'],
 2:['Monday_2022_03_14','Wednesday_2022_03_16','Friday_2022_03_18'],
 3:['Tuesday_2022_03_15','Wednesday_2022_03_16],
 ...}  #and so on

I tried to implement the next solution on a loop (I found it in Get key by value in dictionary), but then I realized this could work only in dicts with keys of only one value, not lists like mine:

list(turns.keys())[list(turns.values()).index(16)]
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
~\AppData\Local\Temp/ipykernel_19168/930684082.py in <module>
----> 1 list(turns.keys())[list(turns.values()).index(16)]

ValueError: 16 is not in list

It’s ok if you don’y provide a loop solution, I can implement it but I need help in getting the key name by value (like the line above), but focusing on a list instead of a simple value.

Thanks to all.

>Solution :

One way is to use dict.setdefault and iterative over the lists:

out = {}
for k, lst in turns.items():
    for v in lst:
        out.setdefault(v, []).append(k)

Then if you want the dictionary sorted by key, you could use sorted:

out = {k: out[k] for k in sorted(out)}

Output:

{1: ['Thursday_2022_03_17'],
 2: ['Monday_2022_03_14', 'Wednesday_2022_03_16', 'Friday_2022_03_18'],
 3: ['Tuesday_2022_03_15', 'Wednesday_2022_03_16'],
 5: ['Thursday_2022_03_17', 'Friday_2022_03_18'],
 6: ['Friday_2022_03_18'],
 7: ['Monday_2022_03_14'],
 9: ['Wednesday_2022_03_16'],
 10: ['Tuesday_2022_03_15'],
 14: ['Wednesday_2022_03_16'],
 16: ['Monday_2022_03_14', 'Tuesday_2022_03_15', 'Thursday_2022_03_17'],
 17: ['Monday_2022_03_14', 'Wednesday_2022_03_16', 'Friday_2022_03_18'],
 19: ['Monday_2022_03_14', 'Wednesday_2022_03_16', 'Thursday_2022_03_17'],
 21: ['Tuesday_2022_03_15', 'Wednesday_2022_03_16'],
 22: ['Tuesday_2022_03_15'],
 23: ['Monday_2022_03_14', 'Thursday_2022_03_17'],
 25: ['Monday_2022_03_14', 'Tuesday_2022_03_15'],
 27: ['Wednesday_2022_03_16', 'Thursday_2022_03_17', 'Friday_2022_03_18'],
 28: ['Tuesday_2022_03_15'],
 29: ['Tuesday_2022_03_15', 'Thursday_2022_03_17', 'Friday_2022_03_18'],
 30: ['Tuesday_2022_03_15', 'Thursday_2022_03_17'],
 31: ['Tuesday_2022_03_15'],
 32: ['Monday_2022_03_14'],
 33: ['Tuesday_2022_03_15'],
 40: ['Monday_2022_03_14', 'Wednesday_2022_03_16', 'Friday_2022_03_18'],
 41: ['Tuesday_2022_03_15', 'Friday_2022_03_18'],
 42: ['Thursday_2022_03_17', 'Friday_2022_03_18'],
 43: ['Monday_2022_03_14', 'Tuesday_2022_03_15'],
 44: ['Wednesday_2022_03_16', 'Thursday_2022_03_17', 'Friday_2022_03_18']}