Inverts a dictionary with non-unique hashable values.

Use dictionary.items() in combination with a loop to map the values of the dictionary to keys using dictionary.setdefault(), list() and append() to create a list for each one.


def collect_dictionary(obj):
  inv_obj = {}
  for key, value in obj.items():
    inv_obj.setdefault(value, list()).append(key)
  return inv_obj


ages = {
  "Peter": 10,
  "Isabel": 10,
  "Anna": 9,
collect_dictionary(ages) # { 10: ["Peter", "Isabel"], 9: ["Anna"] }