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：

---------------------
Shape: Triangular, Boxy, Square, Round
---------------------
Shape: Triangular, Boxy, Square, Round
---------------------
Triangular
Boxy
Square
Round: Ellipse, Circle
---------------------
Triangular
Boxy
Square
Round: Ellipse, Circle

python之元编程（元类实例）