Efficient way to get from {key1: [value1, value2...]} to {value1: [key1, key2...]}

Question

Say I have a dictionary like this:

animals_dict = 
{'cat': ['black', 'white', 'orange'],
'dog': ['black', 'white'],
'parrot': ['orange', 'green']}

And I want to get this:

colors_dict = 
{'black': ['cat', 'dog'],
'white': ['cat', 'dog'],
'orange': ['cat', 'parrot'],
'green': ['parrot']}

The obvious way is to cycle through each animal in animals_dict; cycle through each of its colors; and add the animal to that color in colors_dict. But if you have tens of millions of animals and each animal can have up to a hundred colors, this will get computationally expensive. Is there a more efficient way to achieve this?

Answer 1

index = {'cat': ['black', 'white', 'orange'],'dog': ['black', 'white'],'parrot': ['orange', 'green']}

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

This gives

{
    'black': ['cat', 'dog'], 
    'white': ['cat', 'dog'], 
    'orange': ['cat', 'parrot'], 
    'green': ['parrot']
}

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