input: 
  def insertion_sort(arr):
      """
      Generate a Python function to execute a valid insertion sort on a given list of integers.
      >>> insertion_sort([1, 3, 2, 5, 4])
      [1, 2, 3, 4, 5]
      >>> insertion_sort([6, 7, 8, 9, 10])
      [6, 7, 8, 9, 10]
      """
      for i in range(1, len(arr)):
          key = arr[i]
          j = i - 1
          while j >= 0 and key < arr[j]:
              arr[j + 1] = arr[j]
              j = j - 1
          arr[j + 1] = key
      return arr
  
output:     for i in range(1, len(arr)):
          key = arr[i]
          j = i - 1
          while j >= 0 and key < arr[j]:
              arr[j + 1] = arr[j]
              j = j - 1
          arr[j + 1] = key
      return arr