old_contents: from cursor import Cursor
  class Document:
      def __init__(self):
          self.characters = []
          self.cursor = Cursor(self)
          self.filename = '';
  
      def insert(self, character):
          self.characters.insert(self.cursor.position, character)
          self.cursor += 1
  
      def delete(self):
          del self.characters[self.cursor.position]
  
      def save(self):
          f = open(self.filename, 'w')
          f.write(''.join(self.characters))
          f.close()
  
      def forward(self):
          self.cursor += 1
  
      def back(self):
          self.cursor -= 1
  
new_contents: from cursor import Cursor
  class Document:
      def __init__(self):
          self.characters = []
          self.cursor = Cursor(self)
          self.filename = '';
  
      def insert(self, character):
          self.characters.insert(self.cursor.position, character)
          self.cursor.forward()
  
      def delete(self):
          del self.characters[self.cursor.position]
  
      def save(self):
          f = open(self.filename, 'w')
          f.write(''.join(self.characters))
          f.close()
  
      @property
      def string(self):
          return ''.join(self.characters)
  
subject: 删除移动cursor的方法,添加属性函数string