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snippets/python/s/collect-dictionary.md

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+---
+title: Invert dictionary
+type: snippet
+language: python
+tags: [dictionary]
+cover: working-bee
+dateModified: 2020-11-02T19:27:07+02:00
+---
+
+Inverts a dictionary with non-unique hashable values.
+
+- Create a `collections.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 `dict.append()`.
+- Use `dict()` to convert the `collections.defaultdict` to a regular dictionary.
+
+```py
+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)
+```
+
+```py
+ages = {
+  'Peter': 10,
+  'Isabel': 10,
+  'Anna': 9,
+}
+collect_dictionary(ages) # { 10: ['Peter', 'Isabel'], 9: ['Anna'] }
+```