Shell Sort in Python

def shellSort(arr):
   gap = 0
   while gap < (len(arr) / 3):
       gap = gap * 3 + 1
   
   while gap > 0:
       for i in range(gap, len(arr)):
           j = i
           tmp = arr[i]

           while j >= gap and arr[j - gap] > tmp:
               arr[j] = arr[j - gap]
               j -= gap
           
           arr[j] = tmp

       gap = (gap - 1) / 3

# ----------
# Demo Code
# ----------
array = [7,4,6,2,3,9,1]
print(shellSort(array))