#!/usr/bin/env python                   
#coding=utf-8
class Squares:                          
    def __init__(self,start,stop):     
        self.value = start - 1
        self.stop = stop
    def __iter__(self):
        return  self           
    def next(self):
        if self.value == self.stop:
            raise   StopIteration                                             
        self.value += 1
        return self.value ** 2
for i in Squares(1,5):
    print i,

请问Squares类中的__iter__循环几次！具体这段代码是怎么运作的？