直接看程式碼，有註解

class Node(object):
    """結點"""
    def __init__(self, item):
        self.elem = item
        self.next = None
        self.prev = None


class DoubleLinkList(object):
    """雙鏈表"""
    def __init__(self, node=None):
        self.__head = node

    def is_empty(self):
        """連結串列是否為空"""
        return self.__head == None

    def length(self):
        """連結串列長度"""
        # cur遊標，用來移動遍歷節點
        cur = self.__head
        # count記錄數量
        count = 0
        while cur != None:
            count += 1
            cur = cur.next
        return count

    def travel(self):
        """遍歷整個連結串列"""
        cur = self.__head
        while cur != None:
            print(cur.elem, end=" ")
            cur = cur.next
        print("")

    def add(self, item):
        """連結串列頭部新增元素，頭插法"""
        node = Node(item)
        node.next = self.__head
        self.__head = node
        node.next.prev = node

    def append(self, item):
        """連結串列尾部新增元素, 尾插法"""
        node = Node(item)
        if self.is_empty():
            self.__head = node
        else:
            cur = self.__head
            while cur.next != None:
                cur = cur.next
            cur.next = node
            node.prev = cur

    def insert(self, pos, item):
        """指定位置新增元素
        :param  pos 從0開始
        """
        if pos <= 0:
            self.add(item)
        elif pos > (self.length()-1):
            self.append(item)
        else:
            cur = self.__head
            count = 0
            while count < pos:
                count += 1
                cur = cur.next
            # 當迴圈退出後，cur指向pos位置
            node = Node(item)
            node.next = cur
            node.prev = cur.prev
            cur.prev.next = node
            cur.prev = node
    def remove(self, item):
        """刪除節點"""
        cur = self.__head
        while cur != None:
            if cur.elem == item:
                # 先判斷此結點是否是頭節點
                # 頭節點
                if cur == self.__head:
                    self.__head = cur.next
                    if cur.next:
                        # 判斷連結串列是否只有一個結點
                        cur.next.prev = None
                else:
                    cur.prev.next = cur.next
                    if cur.next:
                        cur.next.prev = cur.prev
                break
            else:
                cur = cur.next
    def search(self, item):
        """查詢節點是否存在"""
        cur = self.__head
        while cur != None:
            if cur.elem == item:
                return True
            else:
                cur = cur.next
        return False
if __name__ == "__main__":
    ll = DoubleLinkList()
    print(ll.is_empty())
    print(ll.length())
    ll.append(1)
    print(ll.is_empty())
    print(ll.length())
    ll.append(2)
    ll.add(8)
    ll.append(3)
    ll.append(4)
    ll.append(5)
    ll.append(6)
    # 8 1 2 3 4 5 6