# Write a Python program to get the difference between two given lists, after applying the provided function to each list element of both.  
 def difference_by(a, b, fn):
  _b = set(map(fn, b))
  return [item for item in a if fn(item) not in _b]
from math import floor
print(difference_by([2.1, 1.2], [2.3, 3.4], floor)) 
print(difference_by([{ 'x': 2 }, { 'x': 1 }], [{ 'x': 1 }], lambda v : v['x']))
