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snippets/python/s/union-by.md

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+---
+title: List union based on function
+type: snippet
+language: python
+tags: [list]
+cover: sunrise-over-city
+dateModified: 2020-11-02T19:28:35+02:00
+---
+
+Returns every element that exists in any of the two lists once, after applying the provided function to each element of both.
+
+- Create a `set` by applying `fn` to each element in `a`.
+- Use a list comprehension in combination with `fn` on `b` to only keep values not contained in the previously created set, `_a`.
+- Finally, create a `set` from the previous result and `a` and transform it into a `list`
+
+```py
+def union_by(a, b, fn):
+  _a = set(map(fn, a))
+  return list(set(a + [item for item in b if fn(item) not in _a]))
+```
+
+```py
+from math import floor
+
+union_by([2.1], [1.2, 2.3], floor) # [2.1, 1.2]
+```