Returns the sum 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 sum() to return the sum of the values.

def sum_by(lst, fn):
  return sum(map(fn, lst))

sum_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda v : v['n']) # 20