Mapped list average

Calculates the average of a list, after mapping each element to a value using the provided function.

• Use map() to map each element to the value returned by fn.
• Use sum() to sum all of the mapped values, divide by len(lst).
• Omit the last argument, fn, to use the default identity function.

def average_by(lst, fn = lambda x: x):
  return sum(map(fn, lst), 0.0) / len(lst)

average_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda x: x['n'])
# 5.0