二叉树的中序遍历-python

def mid_order(root):
    if root == None:
        print("")

    stack = []
    stack.append(root)
    while len(stack) or root != None:
        if root != None:
            stack.append(root.left)  # 只负责进
            root = root.left
        else:
            pop = stack.pop() # 只负责出
            print(pop.val)
            root = root.left