input: Write a function to generate a Python function that can execute a valid insertion sort 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([3, 4, 5, 1, 2]) == [1, 2, 3, 4, 5]
  assert insertion_sort([3, 5, 10, 1, 2]) == [1, 2, 3, 5, 10]
  assert insertion_sort([4, 3, 1, 2]) == [1, 2, 3, 4]
  assert insertion_sort([]) == []
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