input: 
  def insertion_sort(list):
      """Implement an insertion sort algorithm in Python.
      >>> insertion_sort([4, 2, 7, 1, 3])
      [1, 2, 3, 4, 7]
      >>> insertion_sort([9, 8, 7, 6, 5, 4, 3, 2, 1])
      [1, 2, 3, 4, 5, 6, 7, 8, 9]
      """
      for i in range(1, len(list)):
          key = list[i]
          j = i - 1
          while j >= 0 and key < list[j]:
              list[j + 1] = list[j]
              j -= 1
          list[j + 1] = key
      return list
  
output:     for i in range(1, len(list)):
          key = list[i]
          j = i - 1
          while j >= 0 and key < list[j]:
              list[j + 1] = list[j]
              j -= 1
          list[j + 1] = key
      return list