关于python的__len__函数问题

Question

class Echo():
    def __init__(self, name):
        self.name = name
        print("Hello {}!".format(name))
    def __len__(self):
        n=0
        name=self.name
        for i in name:
            if i.isupper()==True:
                n=n+1;
        print("Hello {}!".format(n))
s = input()
echoA = Echo(s)
len(echoA)

想问一下各位,为何会报这个错'NoneType' object cannot be interpreted as an integer

Answer 1

定义魔术方法 def __len__(self) 时，需要返回一个数值，，，即 __len__() should return >= 0。。你这里没有显示使用 return 语句，所以默认返回 None

Answer 2

__len__:官方文档

该方法必须要有返回值，并且返回值为 integer >= 0.

Answer 3

>>> class Echo():
...     def __init__(self, name):
...         self.name = name
...         print("Hello {}!".format(name))
...     def __len__(self):
...         n=0
...         name=self.name
...         for i in name:
...             if i.isupper()==True:
...                 n=n+1;
...         return n
...
>>> s = input()
WWW.OS373.CN
>>> echoA = Echo(s)
Hello WWW.OS373.CN!
>>> len(echoA)
7