How to invert dictionary with multiple values of each key?-CodePudding

I have been working on this forever. I have the following dictionary.

Subject1='Math'
Subject2='English'
Subject3='Chemstry'
Subject4='Physics'
Subject5='Geology'
Subject6='PE'
Subject7='Music'
Subject8='Psychology'
Subject9='Politics'
Subject10='Acting'

dict_1 = {(Subject1, Subject2):['Michael','James','Lydia'],
          (Subject3, Subject4):['Michael','Lydia','James'],
          (Subject5, Subject6):['Tom'],
          (Subject7, Subject8):[],
          (Subject9, Subject10):[]
}

I want to inverse the dictionary so that it will look like:

{['Michael', 'James', 'Lydia']: {(Subject1, Subject2), (Subject3, Subject4)}, 
 ['Tom']: {(Subject5, Subject6)}, 
 []: {(Subject7, Subject8), (Subject9, Subject10)} }

I tried

from collections import defaultdict

new_tel = defaultdict(list)
for key, value in dict_1.items():
    new_tel[value].append(key)

This worked for hashable type but not unhashable type list. Maybe I can join the string of ['Michael','James','Lydia'] to 'James Lydia Michael' so that it can be a valid key? How do I sort the elements in the list ['Michael','James','Lydia'] and ['Michael','Lydia','James'] so that they are in alphabetical order before I join them?

This is a more achievable output:

{'James Lydia Michael': {(Subject1, Subject2), (Subject3, Subject4)}, 
 'Tom': {(Subject5, Subject6)}, 
 'None': {(Subject7, Subject8), (Subject9, Subject10)} }

CodePudding user response：

Dictionary keys can't be lists but it can be tuples. So you can do something like below. Sort the values, make them tuples to use as keys and use dict.setdefault:

out = {}
for k,v in dict_1.items():
    out.setdefault(tuple(sorted(v)), set()).add(k)

Output:

{('James', 'Lydia', 'Michael'): {('Chemstry', 'Physics'), ('Math', 'English')},
 ('Tom',): {('Geology', 'PE')},
 (): {('Music', 'Psychology'), ('Politics', 'Acting')}}

CodePudding user response：

As told in the question comments lists are unhashable. if you can use tuple instead, "Dict Comprehensions" is a good way. More info: https://www.python.org/dev/peps/pep-0274/.

dict_reverse = {tuple(dict_1[key]): key for key in dict_1.keys()}

CodePudding user response：

Easiest would be to use a tuple as key type

for key, value in dict_1.items():
    new_tel[tuple(sorted(value))].append(key)


{('James', 'Lydia', 'Michael'): [('Math', 'English'), ('Chemstry', 'Physics')], 
 ('Tom',): [('Geology', 'PE')],  
 (): [('Music', 'Psychology'), ('Politics', 'Acting')]})

If you join to set the key type as str

for key, value in dict_1.items():
    new_tel[" ".join(sorted(value))].append(key)


{'James Lydia Michael': [('Math', 'English'), ('Chemstry', 'Physics')], 
 'Tom': [('Geology', 'PE')], 
 '': [('Music', 'Psychology'), ('Politics', 'Acting')]})