Returns the symmetric difference between two lists, after applying the provided function to each list element of both.

Create a set by applying fn to each element in every list, then use list comprehension in combination with fn on each one to only keep values not contained in the previously created set of the other.

def symmetric_difference_by(a, b, fn):
  _a, _b = set(map(fn, a)), set(map(fn, b))
  return [item for item in a if fn(item) not in _b] + [item for item in b if fn(item) not in _a]

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