Swap around key and value (containing a list) within a Python dictionary -

i have reference dictionary subjects , page numbers so:

reference = { 'maths': [3, 24],'physics': [4, 9, 12],'chemistry': [1, 3, 15] } 

i need writing function inverts reference. is, returns dictionary page numbers keys, each associated list of subjects. example, swap(reference) run on above example should return

{ 1: ['chemistry'], 3: ['maths', 'chemistry'], 4: ['physics'],  9: ['physics'], 12: ['physics'], 15: ['chemistry'], 24: ['maths'] } 

you can use defaultdict:

from collections import defaultdict  d = defaultdict(list) reference = { 'maths': [3, 24],'physics': [4, 9, 12],'chemistry': [1, 3, 15] } a, b in reference.items():        in b:             d[i].append(a) print(dict(d)) 

output:

{1: ['chemistry'], 3: ['maths', 'chemistry'], 4: ['physics'], 9: ['physics'], 12: ['physics'], 15: ['chemistry'], 24: ['maths']} 

without importing collections:

d = {} a, b in reference.items():     in b:         if in d:            d[i].append(a)         else:            d[i] = [a] 

output:

{1: ['chemistry'], 3: ['maths', 'chemistry'], 4: ['physics'], 9: ['physics'], 12: ['physics'], 15: ['chemistry'], 24: ['maths']}