Pythonic Way to reverse nested dictionaries

Question

I have a nested dictionary of people and item ratings, with people as the key. people may or may not share items. Example:

{
 \'Bob\' : {\'item1\':3, \'item2         


        

Answer 1

I totally agree that Ryan Ginstrom's answer is the preferred way of doing this (for all practical purposes).

But since the question also explicitely asks:

Is it possible with a comprehension?

I thought I'd chime in with a quick example as for how to do this with a list comprehension (it could be a good example for showing how nested list comphrehensions can quickly decrease readability).

import itertools

d = {
 'Bob' : {'item1':3, 'item2':8, 'item3':6},
 'Jim' : {'item1':6, 'item4':7},
 'Amy' : {'item1':6,'item2':5,'item3':9,'item4':2}
}

print dict([(x, dict([(k, d[k][x]) for k,v in d.items() if x in d[k]])) 
            for x in set(itertools.chain(*[z for z in d.values()]))])