python 单例之多种方法

1.__new__方法

class Son(object):

    __instance = None

   

    def __new__(cls,*args,**kwargs):

        if not cls.__instance:

            cls.__instance = object.__new__(cls)

        return cls.__instance

a =Son()

b =Son()

print(id(a))

print(id(b))

2.装饰器方法

def pro(class_):

    info = {}

    def wrappend(*args,**kwargs):

        if class_ not in info:

            info[class_] = class_(*args,**kwargs)

        return info[class_]

    return wrappend

@pro

class Foo():

    pass

a = Foo()

b = Foo()

print(id(a))

print(id(b))

3.类装饰器方法

class Son(object):

    __instance = None

   

    def __init__(self,class_):

        self.class_ = class_

   

    def __call__(self,*args,**kwargs):

        if not Son.__instance:

            Son.__instance = self.class_(*args,**kwargs)

        return Son.__instance

   

@Son

class Foo(object):

    pass

b1 = Foo()

b2 = Foo()

print(id(b1))

print(id(b2))

4.元类方法

class UpperAttrMetaClass(type):

    # init方法也可以实现

    def __init__(self,*args,**kwargs):

        self._instance = None

        type.__init__(*args,**kwargs)

    # new方法可以实现

    #def __new__(cls,class_name,class_parents,class_attr):

    #    class_attr['_instance'] = None

    #    return type.__new__(cls,class_name,class_parents,class_attr)

       

    #    # 下边这个不可取

    #    #return type(class_name,class_parents,class_attr)

   

    def __call__(self,*args,**kwargs):

        if not self._instance:

            self._instance = super().__call__(*args,**kwargs)

        return self._instance

# 在代码执行到class FOO这里的时候 元类的__new__方法 和 __init__方法其实已经被执行

class Foo(object,metaclass=UpperAttrMetaClass):

    pass

f1 = Foo()

f2 = Foo()

print(id(f1))

print(id(f2))