input: Write a python function to calculate the average of a list of numbers.
  assert average_of_period([1, 3, 4, 5, 3, 9, 6]) == 4.428571428571429
  assert average_of_period([10, 20, 30, 40, 50]) == 30
  assert average_of_period([7, 14, 21, 28]) == 17.5
output: def average_of_period(numbers):
      return sum(numbers) / len(numbers) if numbers else 0