def create_class_by_name(name):
    if name == 'dog': 
        class Dog(object):
            pass
        return Dog
    else:
        class Cat(object):
            pass
        return Cat
dy_class = create_class_by_name('hi')
print dy_class    # output: <class '__main__.Cat'>
print dy_class()  # output: <__main__.Cat object at 0x03601D10>

class ClassParent(object):
    first_name = 'John'
# 創(chuàng)建一個(gè) 繼承自 ClassParent 的 ClassChild，并為 ClassChild 添加一個(gè) age = 15 的 attribute
child_class = type('ClassChild', (ClassParent,), {'age': 15})
# child_class 形如：
class ClassChild(ClassParent):
    age = 15
child_obj = child_class()
print child_obj.first_name, child_obj.age
# output: John 15

print child_class.__class__
# output <type 'type'>

class DemoMeta(type):
	pass
class DemoClass(object):
	__metaclass__ = DemoMeta
print type(DemoClass) #<class '__main__.DemoMeta'>

class FooMeta(type):
	def __new__(mcs, name, bases, attrs):
		"""
		定制創(chuàng)建 class 的行為
		作為示例，這里將外部傳入的 attrs 的名稱做一些處理：如果以'_'開頭，則轉(zhuǎn)為小寫
		:param name: class 名稱
		:param bases: tuple, 父類的名稱
		:param attrs: class attributes
		"""
		converted = {atr if not atr.startswith('_') else atr.lower(): v 
                     for atr, v in attrs.items()}
		cls = super(FooMeta, mcs).__new__(mcs, name, bases, converted)
		return cls
class Foo(object):
    __metaclass__ = FooMeta

class Foo(object, metaclass=CustomMetaclass):
    pass

import gc
def count_subclasses(cls):
    count = 0
    for obj in gc.get_objects():
        if isinstance(obj, type) and issubclass(obj, cls):
            count += 1
    return count

class Meta(type):
    registry = {}
    def __init__(cls, name, bases, attrs):
        super(Meta, cls).__init__(name, bases, attrs)
        if name != 'Base':
            Meta.registry[name] = cls
class Base(object):
    __metaclass__ = Meta
class Subclass1(Base):
    pass
class Subclass2(Base):
    pass
print Meta.registry

class Singleton(type):
    def __init__(cls, name, bases, dict):
        super(Singleton, cls).__init__(name, bases, dict)
        cls.instance = None
    def __call__(cls, *args):
        if cls.instance is None:
            cls.instance = super(Singleton, cls).__call__(*args)
        return cls.instance
class MyCard(object):
	__metaclass__ = Singleton
def testSingle():
	card1 = MyCard()
	card2= MyCard()
	print card1,card2
    #輸出結(jié)果：<__main__.MyCard object at 0x03A6FE90> <__main__.MyCard object at 0x03A6FE90>

class Meta(type):
    def __init__(cls, name, bases, attrs):
        super(Meta, cls).__init__(name, bases, attrs)
        cls.name = name
        cls.hello = lambda self: 'Hello, %s!' % self.name
class Base(object):
    __metaclass__ = Meta
class Subclass1(Base):
    pass
class Subclass2(Base):
    pass
print Subclass1().hello()
print Subclass2().hello()
#Hello, Subclass1!
#Hello, Subclass2!