Question

所以我正在为Python编写一个程序，其中一个＆＃34; User＆＃34;对象被创建和操纵。

我有一个对象的链接列表，其中节点代表User个对象。

当我将变量x设置为User对象时，通过搜索列表并将变量设置为等于返回的Node数据，（例如x = List.search("a username")其中search返回具有搜索名称的用户对象）我得到一个User对象（使用type(x)验证）

但是，在尝试使用此User变量上x类的方法时，我收到NoneType错误。

可能导致这种情况的原因是什么？如果x直接分配给User对象（即x = User()），但在前一种情况下无效，则可以正常工作。

代码注释：从.txt文件中获取命令，这些命令是＆＃34;添加＆＃34;和＃34;朋友＆＃34;等等。朋友应该将每个User对象添加到另一个User对象的LinkedList中，反之亦然

代码：

class Node (object):

   def __init__(self,initdata):
      self.data = initdata
      self.next = None            # always do this â€“ saves a lot
                                  # of headaches later!
   def getData (self):
      return self.data            # returns a POINTER

   def getNext (self):
      return self.next            # returns a POINTER

   def setData (self, newData):
      self.data = newData         # changes a POINTER

   def setNext (self,newNext):
      self.next = newNext         # changes a POINTER

class UnorderedList ():

   def __init__(self):
      self.head = None

   def isEmpty (self):
      return self.head == None

   def add (self,item):
      # add a new Node to the beginning of an existing list
      temp = Node(item)
      temp.setNext(self.head)
      self.head = temp

   def length (self):
      current = self.head
      count = 0

      while current != None:
         count += 1
         current = current.getNext()

      return count

   def search (self,item): #NOW RETURNS OBJECT IN LIST
      current = self.head

      while current != None:
         if current.getData().name == item:
             return current.getData()
         else:
            current = current.getNext()

      return None

   def remove (self,item):
      current = self.head
      previous = None
      found = False

      while not found:
         if current.getData() == item:
            found = True
         else:
            previous = current
            current = current.getNext()

      if previous == None:
         self.head = current.getNext()
      else:
         previous.setNext(current.getNext() )

class User():

    def __init__(self):
        self.name = ""
        self.friendsList = UnorderedList()

    def setName(self,info):
        self.name = info

    def getName(self):
        return self.name

    def removeFriend(self, item):
        self.friendsList.remove(item)

    def addFriend(self, item):
        self.friendsList.add(item)

    def searchList(self, item):
        self.friendsList.search(item)

    def __str__(self):
        return self.name

def main():

    inFile = open("FriendData.txt")
    peopleList = UnorderedList()

    for line in inFile:
        textList = line.split()

        if "Person" in textList:

            newUser = User()
            newUser.setName(textList[1])

            if peopleList.search(newUser.getName()) != None:
                print("This user is already in the program")

            else:
                peopleList.add(newUser)
                print(newUser.getName(),"now has an account")

        elif "Friend" in textList:
            #PROBLEM OBJECTS a AND b BELOW
            a = peopleList.search(textList[1]) #returns user1 object
            b = peopleList.search(textList[2]) # return user2 object
            b.getName()


            if peopleList.search(textList[1]) == None:
                print("A person with the name", textList[1], "does not currently exist")
            elif peopleList.search(textList[2]) == None:
                print("A person with the name", textList[2], "does not currently exist")
            elif textList[1]==textList[2]:
                print("A person cannot friend him/herself")
            elif peopleList.search(textList[1]).searchList(textList[2])!= None:
                print(textList[1],"and",textList[2],"are already friends")
            elif peopleList.search(textList[2]).searchList(textList[1]) != None:
                print(textList[2],"and",textList[1],"are already friends")
            #else:
                #a.friendsList.add(b) #adds user 2 to user1 friendlist
                #b.addFriend(a)
                #print(a.getName(),"and",b.getName(),"are now friends")

main()

错误：

Traceback (most recent call last):
  File "C:/Users/rsaen/Desktop/Python Prgms/Friends.py", line 137, in <module>
    main()
  File "C:/Users/rsaen/Desktop/Python Prgms/Friends.py", line 119, in main
    b.getName()
AttributeError: 'NoneType' object has no attribute 'getName'

解析.txt文件

['Person', 'Rachel']
['Person', 'Monica']
['Friend', 'Rachel', 'Monica']
['Friend', 'Monica', 'Rachel']
['Friend', 'Rachel', 'Ross']
['Person', 'Ross']
['Friend', 'Rachel', 'Ross']
['Person', 'Joey']
['Person', 'Joey']
['Friend', 'Joey', 'Joey']
['Friend', 'Joey', 'Rachel']
['Person', 'Chandler']
['Friend', 'Chandler', 'Monica']
['Friend', 'Chandler', 'Rachel']
['Friend', 'Ross', 'Chandler']
['Friend', 'Phoebe', 'Rachel']
['Person', 'Phoebe']
['Friend', 'Phoebe', 'Rachel']
['Exit']

Answer 1

从代码和错误消息中看起来好像textList[2]属性中没有friendsList。事实上，它看起来不会;看起来User属性中唯一的friendsList将是textList[1]，因为friendsList在没有任何参数的情况下被实例化，之后只会添加textList[1]。< / p>

重要的是要知道：Python中没有指针。有关详情，请参阅此处：https://ubuntuforums.org/showthread.php?t=895804

同样重要的是要知道：以下是如何在Python中测试对象是否为None：

if x is not None:
    print(x)

Answer 2

问题在于，在你输入的第5行，你试图在添加Ross之前添加Rachel和Ross之间的友谊。该程序找不到Ross，变量b为None，b.getName（）崩溃。

我注意到两件事：在User类的搜索方法中，您需要从搜索中返回结果。我也看到你使用的是getter和setter，它们可能是赋值的一部分（？），但Python中不需要这些，因为它有一种更温和的封装形式。

说对象是NoneType时出错？

2 个答案: