input: Write a function to implement the insertion sort algorithm on a given list of integers. The function should return the sorted list.
  
  def insertion_sort(arr):
   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
  
  assert insertion_sort([4, 3, 2, 10, 12, 1, 5, 6])==[1, 2, 3, 4, 5, 6, 10, 12]
  assert insertion_sort([5, 2, 9, 1, 5, 6])==[1, 2, 5, 5, 6, 9]
  assert insertion_sort([0, 0, 0, 0])==[0, 0, 0, 0]
  assert insertion_sort([])==[]
  assert insertion_sort([1])==[1]
output: def insertion_sort(arr):
   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