Add collect_dictionary

snippets/collect_dictionary.md

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+---
+title: collect_dictionary
+tags: dictionary,intermediate
+---
+
+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.
+
+```py
+def collect_dictionary(obj):
+  inv_obj = {}
+  for key, value in obj.items():
+    inv_obj.setdefault(value, list()).append(key)
+  return inv_obj
+```
+
+```py
+ages = {
+  "Peter": 10,
+  "Isabel": 10,
+  "Anna": 9,
+}
+collect_dictionary(ages) # { 10: ["Peter", "Isabel"], 9: ["Anna"] }
+```