1.單例模式

　　類外裝飾器實現單例模式，是攔截整個實例化過程。（__new__;__init__）

def singleton(cls):
    _instance = {}
    def func(*args,**kwargs):
        if cls not in _instance:
            _instance[cls] = cls(*args,**kwargs)
        return _instance[cls]
    return func

@singleton
class Test(object):
    def __init__(self,name):
        self.name 
 
= name

if __name__ == ‘__main__‘:
    a = Test(‘BOB‘)
    b = Test(‘JON‘)

    print(a is b)
    print(id(a),id(b))

在上面，我們定義了一個裝飾器 singleton，它返回了一個內部函數 func，
該函數會判斷某個類是否在字典 _instances 中，如果不存在，則會將 cls 作為 key，cls(*args, **kwargs) 作為 value 存到 _instances 中，
否則，直接返回 _instances[cls]。
　

class Singleton(object):
    _instance 
 
= None

    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = object.__new__(cls,*args,**kwargs)
        return cls._instance


class Test(Singleton):

    def __init__(self):
        self.name = ‘bob‘


if __name__ == ‘__main__‘:
    a = Test()
    b = Test()
    
    
 
print(a is b)

*************************************************************************************************************************************************

# instance_dict = {}
#
# class B(object):
#     __float = 0
#
#     def __new__(cls, *args, **kwargs):
#         if cls not in instance_dict:
#             instance_dict[cls] = object.__new__(cls, *args, **kwargs)
#
#         return instance_dict[cls]
#
#     def __init__(self, name):
#         if self.__float == 0:
#             self.my_name = name
#             self.__float = 1
#
#
# if __name__ == ‘__main__‘:
#     a = B(1)
#     b = B(2)
#     print a.my_name, b.my_name

類內實現單例模式，只攔截了__new__生成對象的過程，沒有攔截初始化實例屬性的過程

攔截__new__方法實現單例模式，新式類為主

python-單例模式&工廠模式