本实例是元类实例，功能是记录该的子类的类名，并以树状结构展示子类的类名。

RegisterClasses继承自type，提供的功能是在__init__接口，为类创建了childrens的集合，并类名保存到对应的父类元组的childrens的集合中。

同时对__str__打印方法和__iter__迭代方法进行了定义，其中：

__iter__方法返回类名的childrens集合，并对其中的元素进行输入。

而Sharp继承自RegisterClasses， 当 for s in Sharp: print s，则会调用继承来的__iter__方法。

class Round(Shape): pass class Square(Shape): pass class Triangular(Shape): pass class Boxy(Shape): pass 上述操作中，Round,Square,Triangular,Boxy均初始化一个childrens的集合， 同时，均将自身的类名添加到Shape元类的childrens的集合里。 class Circle(Round): pass class Ellipse(Round): pass

上述操作中，Circle，Ellipse均初始化一个childrens的集合，

同时，均将自身的类名添加到Round，Shape的childrens的集合里，

原因是：Round的父类是Shape，故Circle，Ellipse均有两个父类。

代码如下：

#!/usr/bin/env python # encoding: utf-8 """ @author: @contract: @file: homework5_1.py @time: 2016/10/26 14:22 """ class RegisterClasses(type):     def __init__(cls, name, bases, atts):         super(RegisterClasses, cls).__init__(name, bases, atts)         #创建一个集合，这样继承元类，都会有一个childrens的集合         cls.childrens = set()         #将把当前的子类保存到父类中去         for base in bases:             if hasattr(base, 'childrens'):                 base.childrens.add(cls)     #classmethod, called on class object     def __iter__(cls):         return iter(cls.childrens)     def __str__(cls):         if len(cls.childrens) > 0:             return cls.__name__ + ": " + ", ".join([sc.__name__ for sc in cls])         else:             return cls.__name__ class Shape(object):     __metaclass__ = RegisterClasses print "---------------------" class Round(Shape): pass class Square(Shape): pass class Triangular(Shape): pass class Boxy(Shape): pass print Shape print "---------------------" class Circle(Round): pass class Ellipse(Round): pass print Shape print "---------------------" for s in Shape: #Iterator over subclasses (def __str__(cls):)     print s print "---------------------" for cls in Shape.childrens:     if len(cls.childrens) > 0:         print cls.__name__ + ": " + ", ".join([sc.__name__ for sc in cls])     else:         print cls.__name__ 输出如下：

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Shape: Triangular, Boxy, Square, Round

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Shape: Triangular, Boxy, Square, Round

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Triangular

Boxy

Square

Round: Ellipse, Circle

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Triangular

Boxy

Square

Round: Ellipse, Circle