30-seconds-of-code/snippets/collect_dictionary.md

title, tags

title	tags
collect_dictionary	dictionary,intermediate

Inverts a dictionary with non-unique hashable values.

• Create a defaultdict with list as the default value for each key.
• Use dictionary.items() in combination with a loop to map the values of the dictionary to keys using append().
• Use dict() to convert the defaultdict to a regular dictionary.

from collections import defaultdict

def collect_dictionary(obj):
  inv_obj = defaultdict(list)
  for key, value in obj.items():
    inv_obj[value].append(key)
  return dict(inv_obj)

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