解读python 内置函数，__getitem__(),__next__(),__iter__()

1.__getitem__()

#!/usr/bin/python3.8.12
# -*- coding: utf-8 -*-
# @Time    : 2022/4/16 上午10:21
# @Author  : pf
# @Software: PyCharm


class MutiplyByTwo:
    def __init__(self,number):
        self.numbers=numbers
        self.counter=0


    def __getitem__(self,index):
        input_dict = {
            'number':self.numbers[index],
            'counter':self.counter}
        return input_dict


if __name__=="__main__":
    numbers=[1,2,3]
    MutiplyByTwo=MutiplyByTwo(numbers)
    for num in MutiplyByTwo:
        print(num)

输出结果

说明：__getitem__方法在类的实例被迭代的时候会被调用，也就是说，在for循环中，会先判断MutiplyByTwo是不是一个可迭代对象，比如list,dict,trule,set,这些都是可迭代对象。然后会根据迭代索引index返回数据，注意没有index系统会报错，index相当于一个计数器。

2. __next__()

#!/usr/bin/python3.8.12
# -*- coding: utf-8 -*-
# @Time    : 2022/4/16 上午10:21
# @Author  : pf
# @Software: PyCharm


class MutiplyByTwo:
    def __init__(self,numbers):
        self.numbers=numbers
        self.counter=0

    def __iter__(self):
        print("__iter__")
        return self

    def __next__(self):
        self.counter +=1
        if self.counter >= self.numbers:
            raise StopIteration()
        return self.counter


if __name__=="__main__":
    numbers=10
    MutiplyByTwo=MutiplyByTwo(numbers)
    for i in MutiplyByTwo:
        print(i)

结果

 每次for循环的时候都会调用__next__()，而__iter__()只是在for第一次被调用，并且__iter__必须返回self,否则next()不起作用，可以看出__getitem__(self,index)是__next__(self)和__iter__(self)的组合。

3.__call__()

class Next:
    List = []

    def __init__(self, low, high):
        for Num in range(low, high):
            self.List.append(Num ** 2)

    def __call__(self, Nu):
        return self.List[Nu]

b = Next(1,7)
print (b.List)
print(b(2))

结果

class Next:
    List = []

    def __init__(self, low, high):
        for Num in range(low, high):
            self.List.append(Num ** 2)

    # def __call__(self, Nu):
    #     return self.List[Nu]

b = Next(1,7)
print(b.List)
print(b(2))

结果 

 说明实例不能直接调用List成员