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python/snippets/combine-values.md

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+---
+title: Combine dictionary values
+type: snippet
+tags: [dictionary]
+cover: fruit-feast
+dateModified: 2021-04-04T14:32:35+03:00
+---
+
+Combines two or more dictionaries, creating a list of values for each key.
+
+- Create a new `collections.defaultdict` with `list` as the default value for each key and loop over `dicts`.
+- Use `dict.append()` to map the values of the dictionary to keys.
+- Use `dict()` to convert the `collections.defaultdict` to a regular dictionary.
+
+```py
+from collections import defaultdict
+
+def combine_values(*dicts):
+  res = defaultdict(list)
+  for d in dicts:
+    for key in d:
+      res[key].append(d[key])
+  return dict(res)
+```
+
+```py
+d1 = {'a': 1, 'b': 'foo', 'c': 400}
+d2 = {'a': 3, 'b': 200, 'd': 400}
+
+combine_values(d1, d2) # {'a': [1, 3], 'b': ['foo', 200], 'c': [400], 'd': [400]}
+```