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

title, tags, expertise, cover, firstSeen, lastUpdated

title	tags	expertise	cover	firstSeen	lastUpdated
List union based on function	list	intermediate	blog_images/sunrise-over-city.jpg	2019-08-21T09:10:39+03:00	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

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]))

from math import floor

union_by([2.1], [1.2, 2.3], floor) # [2.1, 1.2]