python——为什么最新的；附加“；在我的双链接列表中没有效果？

类节点：

def __init__(self,item,prev,next):

    self.item = item
    self.prev = prev
    self.next = next

德克级：

def __init__(self,name):

    self.name = name
    self.head = Node(None,None,None)
    self.tail = Node(None,self.head,None)

    self.head.next = self.tail
    self.size = 0

def append(self,item):
    current = self.head
    new_node = Node(item,None,None)
    while current.next != None:
        current = current.next
    current.next = new_node
    new_node.prev = current
    self.tail = new_node
    self.size += 1
    return

    
def appendleft(self,item):
    current = self.tail
    new_node = Node(item,None,None)
    while current.prev != None:
        current = current.prev
    current.prev = new_node
    new_node.next = current
    self.head = new_node
    self.size += 1
    return

def print_list(self): 

    p = self.head

    while p.next != None:
        if p.item == None:
            pass
        else:
            print(p.item,end=' ')
            
        p = p.next

def Group_of_append(self):
    print('Group_of_append')
    self.append('graphe')
    self.append('cherry')
    self.append('apple')
    self.appendleft('watermelon')
    self.appendleft('strawberry')
    self.print_list()
    print(" ")

而且最新的附加代码总是有效果的。例如

self.append('apple1')
self.append('apple2')
self.append('apple3')
self.append('apple4')

结果：“；apple1 apple2 apple3“；