Example of python implementation of bi-directional circular chain table operations in data structures

class Node:
  def __init__(self, elem):
     = elem
     = None
     = None


class DoubleCycleLinkList:
  def __init__(self, node=None):
    self.__head = node

  def is_empty(self):
    """Judgement of emptiness."""
    if self.__head is None:
      return True
    return False

  def length(self):
    """Length of chain table"""
    if self.is_empty():
      return 0
    cur = self.__head
    count = 1
    while  is not self.__head:
      count += 1
      cur = 
    return count

  def travel(self):
    """Traversing a chained table."""
    if self.is_empty():
      return
    cur = self.__head
    while  is not self.__head:
      print(, end=" ")
      cur = 
    print(, end=" ")
    print("")

  def add(self, elem):
    """Head-in-the-sand."""
    node = Node(elem)
    if self.is_empty():
      self.__head = node
       = node
       = node
    else:
      self.__head. = node
       = self.__head.prev
       = self.__head
      self.__head.prev = node
      self.__head = node

  def append(self, elem):
    """Tail insertion."""
    node = Node(elem)
    if self.is_empty():
      self.__head = node
       = node
       = node
    else:
       = self.__head
       = self.__head.prev
      self.__head. = node
      self.__head.prev = node

  def insert(self, pos, elem):
    """Insert at any position (pos), counting from zero."""
    if pos <= 0:
      (elem)
    elif pos > (() - 1):
      (elem)
    else:
      count = 0
      cur = self.__head
      node = Node(elem)
      while count < (pos - 1):
        count += 1
        cur = 
       = 
       = cur
       = node
       = node

  def remove(self, elem):
    """Delete a node, if there is more than one eligible node, just delete the first one"""
    if self.is_empty():
      return
    cur = self.__head
    while  is not self.__head:
      if  == elem:
        if cur is self.__head:
          self.__head = 
           = 
           = 
        else:
           = 
           = 
        break
      cur = 
    if  == elem:
       = self.__head
       = 

  def search(self, elem):
    """Find a node."""
    if self.is_empty():
      return False
    cur = self.__head
    while  is not self.__head:
      if  == elem:
        return True
      cur = 
    # while can't handle the tail node, so do the last tail node judgment
    if  == elem:
      return True
    return False