Python类型在返回前显示实例，但不显示

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]

此输出类型为“instance”

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

nodeHead = builder.buildTree(nodeList)

print type(nodeHead)

当我打印生成器返回的类型时，我得到“type'NoneType'”打印出来

那么为什么它在返回之前是一个有效实例，而不是在返回之后呢

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被None值填充。通过更改buildTree函数，使其

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