30 seconds of python

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Returns the 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 b, then use list comprehension in combination with fn on a to only keep values not contained in the previously created set, _b.

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