Returns the maximum value of a list, after mapping each element to a value using the provided function.

Use map() with fn to map each element to a value using the provided function, use max() to return the maximum value.

def max_by(lst, fn):
  return max(map(fn, lst))

max_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda v : v['n']) # 8