python双向冒泡

双向冒泡：①从左往右，交换相邻的元素，从而达到循环一边可以把最大的元素放在右边；②从右往左进行冒泡，从而把小的元素放在左边。循环直至排序结束。
用代码块功能插入代码，请勿粘贴截图
def bubbleSort(alist):
    for passnum in range(len(alist)-1, 0, -1):
        for i in range(passnum):
            if alist[i] > alist[i+1]:
                alist[i], alist[i+1] = alist[i+1], alist[i]

alist = [54, 26, 93, 17, 77, 31, 44, 55, 20]
bubbleSort(alist)
print(alist)


def shortBubbleSort(alist):
    exchanges = True
    passnum = len(alist)-1
    while passnum > 0 and exchanges:
       exchanges = False
       for i in range(passnum):
           if alist[i]>alist[i+1]:
               exchanges = True
               alist[i], alist[i + 1] = alist[i + 1], alist[i]
       passnum = passnum-1


alist = [20, 30, 40, 90, 50, 60, 70, 80, 100, 110]
shortBubbleSort(alist)
print(alist)
我的解答思路和尝试过的方法
我想要达到的结果双向冒泡：①从左往右，交换相邻的元素，从而达到循环一边可以把最大的元素放在右边；②从右往左进行冒泡，从而把小的元素放在左边。循环直至排序结束。
def biBubbleSort(alist):
    exchanges = True
    l = len(alist)
    passnum = l-1
    while passnum > l//2 and exchanges:
        exchanges = False
        for i in range(l-passnum-1, passnum):
            if alist[i]>alist[i+1]:
                exchanges = True
                alist[i], alist[i + 1] = alist[i + 1], alist[i]
            j = l-i-1
            if alist[j-1]>alist[j]:
                exchanges = True
                alist[j], alist[j-1] = alist[j-1], alist[j]
        passnum = passnum-1