python之元编程（元类实例）

python之元编程（元类实例）
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。