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__ = RegisterClassesprint "---------------------"
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
