Returns a list of elements that exist in both lists, after applying the provided function to each list element of both.

Create a set by applying fn to each element in b, then use list comprehension in combination with fn on a to only keep values contained in both lists.

def intersection_by(a, b, fn):
  _b = set(map(fn, b))
  return [item for item in a if fn(item) in _b]

from math import floor
intersection_by([2.1, 1.2], [2.3, 3.4],floor) # [2.1]