为什么会出现“实例没有'__getitem__'属性”错误？

Question

这是代码：

class BinaryTree:
    def __init__(self,rootObj):
        self.key = rootObj
        self.left = None
        self.right = None
        root = [self.key, self.left, self.right]

    def getRootVal(root):
        return root[0]

    def setRootVal(newVal):
        root[0] = newVal

    def getLeftChild(root):
        return root[1]

    def getRightChild(root):
        return root[2]

    def insertLeft(self,newNode):
        if self.left == None:
                self.left = BinaryTree(newNode)
        else:
            t = BinaryTree(newNode)
            t.left = self.left
            self.left = t

    def insertRight(self,newNode):
        if self.right == None:
            self.right = BinaryTree(newNode)
        else:
            t = BinaryTree(newNode)
            t.right = self.right
            self.right = t

def buildParseTree(fpexp):
    fplist = fpexp.split()
    pStack = Stack()
    eTree = BinaryTree('')
    pStack.push(eTree)
    currentTree = eTree
    for i in fplist:
        if i == '(':
            currentTree.insertLeft('')
            pStack.push(currentTree)
            currentTree = currentTree.getLeftChild()
        elif i not in '+-*/)':
            currentTree.setRootVal(eval(i))
            parent = pStack.pop()
            currentTree = parent
        elif i in '+-*/':
            currentTree.setRootVal(i)
            currentTree.insertRight('')
            pStack.push(currentTree)
            currentTree = currentTree.getRightChild()
        elif i == ')':
            currentTree = pStack.pop()
        else:
            print "error:  I don't recognize " + i
    return eTree

def postorder(tree):
    if tree != None:
        postorder(tree.getLeftChild())
        postorder(tree.getRightChild())
        print tree.getRootVal()

def preorder(self):
    print self.key
    if self.left:
        self.left.preorder()
    if self.right:
        self.right.preorder()

def inorder(tree):
    if tree != None:
        inorder(tree.getLeftChild())
        print tree.getRootVal()
        inorder(tree.getRightChild())

class Stack:
    def __init__(self):
        self.items = []

    def isEmpty(self):
        return self.items == []

    def push(self, item):
        self.items.append(item)

    def pop(self):
        return self.items.pop()

    def peek(self):
        return self.items[len(self.items)-1]

    def size(self):
        return len(self.items)

def main():
    parseData = raw_input( "Please enter the problem you wished parsed.(NOTE: problem must have parenthesis to seperate each binary grouping and must be spaced out.) " )
    tree = buildParseTree(parseData)
    print( "The post order is: ", + postorder(tree))
    print( "The post order is: ", + postorder(tree))
    print( "The post order is: ", + preorder(tree))
    print( "The post order is: ", + inorder(tree))

main()

这里是错误信息：

Please enter the problem you wished parsed.(NOTE: problem must have parenthesis to seperate each binary grouping and must be spaced out.) ( 1 + 2 )
Traceback (most recent call last):
  File "C:\Users\Kevin\Desktop\Python Stuff\Assignment 11\parseTree.py", line 108, in 
    main()
  File "C:\Users\Kevin\Desktop\Python Stuff\Assignment 11\parseTree.py", line 102, in main
    tree = buildParseTree(parseData)
  File "C:\Users\Kevin\Desktop\Python Stuff\Assignment 11\parseTree.py", line 46, in buildParseTree
    currentTree = currentTree.getLeftChild()
  File "C:\Users\Kevin\Desktop\Python Stuff\Assignment 11\parseTree.py", line 15, in getLeftChild
    return root[1]
AttributeError: BinaryTree instance has no attribute '__getitem__'

Answer 1

正如PreludeAndFugue所说，你需要修正格式。不过从我了解到的情况来看，你的BinaryTree类的方法里至少有一个错误：大部分方法根本没有把self作为参数。修正这个问题后，情况可能会好一些。

Answer 2

你第一个问题是，root 应该改成 self.root。

你第二个问题是在这里：

def getLeftChild(root):
    return root[1]

你在这里给 root 赋予了一个新的含义。

Answer 3

因为你声明方法的方式不对：

我们来看看当你调用 tree.getRootVal() 时会发生什么。.getRootVal() 是这样声明的：

def getRootVal(root):
    return root[0]

你可能知道，传给方法的第一个参数总是实例，而且这个参数是隐式提供的。所以你实际上是在把一个 BinaryTree 的实例当成一个序列来处理（root[0]）。

你需要这样指定：

class BinaryTree:
    def __init__(self,rootObj):
        self.key = rootObj
        self.left = None
        self.right = None
        self.root = [self.key, self.left, self.right]   # self.root

    def getRootVal(self):
        return self.root[0]   # access self.root

    def setRootVal(self, newVal):
        self.root[0] = newVal

    # and also the other functions

对象方法的第一个参数不一定要叫 self，但这样命名有助于避免像你遇到的那种错误。

有趣的是，你的 insertLeft 和 insertRight 方法声明得是正确的；）