input: 
  def bubble_sort(arr): 
      """ Sort a given array in ascending order using bubble sort.
      >>> bubble_sort([3, 7, 4, 1])
      [1, 3, 4, 7]
      """
      n = len(arr) 
      for i in range(n-1):
          for j in range(0, n-i-1): 
              if arr[j] > arr[j+1] : 
                  arr[j], arr[j+1] = arr[j+1], arr[j] 
      return arr
  
output:     n = len(arr) 
      for i in range(n-1):
          for j in range(0, n-i-1): 
              if arr[j] > arr[j+1] : 
                  arr[j], arr[j+1] = arr[j+1], arr[j] 
      return arr