input: Write a function to implement the insertion sort algorithm in Python. The function should take a list of numbers as input and return the list sorted in non-decreasing order.
  
  def insertion_sort(list):
      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
  
  assert insertion_sort([3, 2, 1]) == [1, 2, 3]
  assert insertion_sort([5, 2, 9, 1, 5, 6]) == [1, 2, 5, 5, 6, 9]
  assert insertion_sort([]) == []
  assert insertion_sort([1]) == [1]
  assert insertion_sort([2, 1]) == [1, 2]
output: def insertion_sort(list):
      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