30 seconds of python

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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, then use 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]))