Python类型在返回之前显示实例但之后没有显示

Question

我有一个用于从一组节点构建树的函数

def buildTree(self, nodeList,):
        if len(nodeList) > 1:
            nodeList = sorted(nodeList, key=self.getKey)
            newNode = self.makeNode(nodeList[0], nodeList[1])
            nodeList.pop(0)
            nodeList.pop(0)
            nodeList.append(newNode)

            nodeList = self.buildTree(nodeList)
        else:
            print type(nodeList[0])
            return nodeList[0]

输出类型＆＃39;实例＆＃39;

from TreeFactory import TreeFactory
nodeList = builder.buildNodeList(
    'the quick brown fox jumped over the lazy dog')

nodeHead = builder.buildTree(nodeList)

print type(nodeHead)

当我从构建器打印返回类型时，我得到＆＃34;键入＆＃39; NoneType＆＃39;＆＃34;打印出来

那么为什么它在返回之前是一个有效的实例，而不是之后呢？

Answer 1

根据https://stackoverflow.com/users/367273/npe，问题是if分支没有返回任何内容，因此递归导致nodeList填充None值。通过更改buildTree函数使其读为

def buildTree(self, nodeList,):
        if len(nodeList) > 1:
            nodeList = sorted(nodeList, key=self.getKey)
            newNode = self.makeNode(nodeList[0], nodeList[1])
            nodeList.pop(0)
            nodeList.pop(0)
            nodeList.append(newNode)

            nodeList = self.buildTree(nodeList)
            return nodeList
        else:
            return nodeList[0]

解决了这个问题。