Python Quick Sort

#the purpose of povit is to divide the array into two partition,
#so that all left <= pivot < all right,
#, and the partition will do all the inplace sort

def print_array(arr):
    for a in arr:
        print a,

def swap(a,index1, index2):
    temp = a[index1]
    a[index1] = a[index2]
    a[index2] = temp

def partition(arr, left, right):
    print "left:"+str(left)
    print "right:"+str(right)
    if right == left:
        return -1

    i = left - 1
    j = left
    pivot = arr[right]
    while j < right:
        if arr[j] > pivot:
            j = j + 1
        elif arr[j] <= pivot:
            i = i + 1
            swap(arr,i,j)
            j = j + 1

    print "swap (left,right):"+str(left)+","+str(right)
    print "before swap"
    print_array(arr)
    swap(arr,i+1,right)
    print "after swap" #weird bug! one more element here?
    print_array(arr)

    #now the povit has divided the array into two partition, 
    #: all left <= pivot < all right
    #need to return the new right for the new left partition
    if i == left-1: return left 
    else: return i
    

def quick_sort(arr, left, right):
    middle = partition(arr,left,right)
    print middle
    if middle != -1:
        print "after quick sort left"
        quick_sort(arr,left,middle)
        print "after quick sort right"
        quick_sort(arr,middle+1,right)



if __name__ == "__main__":
    array = [9,4,1,2,5,3,6,8,7,0]
    quick_sort(array,0,9)
    print_array(array)