Inverting a dictionary with list values

Question

So, I have this index as a dict.

index = {\'Testfil2.txt\': [\'nisse\', \'hue\', \'abe\', \'pind\'], \'Testfil1.txt\': [\'hue\', \'abe\', 
\'tosse\', \'svend         


        

Answer 1

My solution for reversing a dictionary. However, it creates a new dictionary new_dic:

new_dic = {}
for k,v in index.items():
    for x in v:
        new_dic.setdefault(x,[]).append(k)

Output :

{'tosse': ['Testfil1.txt'], 'nisse': ['Testfil2.txt'], 'svend': ['Testfil1.txt'], 'abe': ['Testfil1.txt', 'Testfil2.txt'], 'pind': ['Testfil2.txt'], 'hue': ['Testfil1.txt', 'Testfil2.txt']}

Answer 2

I've tried around and you want to use val not in inverse but it can't be checked if a "list is in a dict". (val is a list)

For your code a simple change will do what you want:

def invert_dict(d): 
    inverse = dict() 
    for key in d: 
        # Go through the list that is saved in the dict:
        for item in d[key]:
            # Check if in the inverted dict the key exists
            if item not in inverse: 
                # If not create a new list
                inverse[item] = [key] 
            else: 
                inverse[item].append(key) 
    return inverse

Answer 3

You can not use list objects as dictionary keys, since they should be hashable objects. You can loop over your items and use dict.setdefault method to create the expected result:

>>> new = {}
>>> 
>>> for k,value in index.items():
...     for v in value:
...         new.setdefault(v,[]).append(k)
... 
>>> new
{'hue': ['Testfil2.txt', 'Testfil1.txt'], 'svend': ['Testfil1.txt'], 'abe': ['Testfil2.txt', 'Testfil1.txt'], 'tosse': ['Testfil1.txt'], 'pind': ['Testfil2.txt'], 'nisse': ['Testfil2.txt']}

and if you are dealing with larger datasets for refusing of calling creating an empty list at each calling the setdefault() method you can use collections.defaultdict() which will calls the missing function just when it encounter a new key.

from collections import defaultdict

new = defaultdict(list)
for k,value in index.items():
    for v in value:
        new[v].append(k)

>>> new
defaultdict(<type 'list'>, {'hue': ['Testfil2.txt', 'Testfil1.txt'], 'svend': ['Testfil1.txt'], 'abe': ['Testfil2.txt', 'Testfil1.txt'], 'tosse': ['Testfil1.txt'], 'pind': ['Testfil2.txt'], 'nisse': ['Testfil2.txt']})