python使用元类实现代码复用

对象是由 __new__() 和 __init__() 协作构造完成的

由 __new__() 创建

由 __init__() 定制  

所以 __init__() 返回的值只能是 None，否则会在运行时引发 TypeError

__new__() 是一个静态方法 (因为是特例所以你不需要显式地声明)

它会将所请求实例所属的类作为第一个参数。其余的参数会被传递给对象构造器表达式 (对类的调用) __new__() 的返回值应为新对象实例 (通常是 cls 的实例)

注意 py2 与 py3， 没有unbounded method in Python 3

py2 重写的 __new__ 需要 @staticmethod装饰器

Why staticmethod decorator not needed? docs.python.org/3/howto/descriptor.html still refers to “unbound methods”

python3-cookbook

通过元类实现单例模式

改变实例创建方式来实现单例、缓存或其他类似的特性

通过重载new方法实现单例模式 （一个类的单例模式）

class Single(object):

    _instance = None

    def __new__(cls, *args, **kwargs):
        if cls._instance:
            return  cls._instance

        cls._instance = cv = super(Single, cls).__new__(cls, *args, **kwargs)
        return cv



a = Single()
b = Single()

print(a is b )

通过元类， 代码复用 (多个类的单例模式)

# 1 复写 __call__ 方法
class SingleMeta(type):


    def __init__(cls, *args, **kwargs):
        print("cls")
        cls._instance = None
        super(SingleMeta, cls).__init__(*args, **kwargs)


    def __call__(cls, *args, **kwargs):
        if cls._instance is None:
            return cls._instance
        cls._instance = super(SingleMeta, cls).__call__(*args, **kwargs)

        return cls._instance

# 1 class A(object, metaclass=SingleMeta) , 元类__init__, 创建 A类对象
# 2 A()，调用 元类的 __call__ 方法， __new__一个新对象


class A(object, metaclass=SingleMeta):
    ...


a = A()
b = A()

print(a is b)



# 2 重写 __new__ 方法  cls.__new__ = new_instance

class SingleMeta(type):
    def __init__(cls, *args):
        cls._instance = None
        origin_new = cls.__new__

        @staticmethod
        def new_instance(cls, *args, **kwargs):
            if cls._instance:
                return cls._instance
            cls._instance = cv = origin_new(cls)
            return cv

        cls.__new__ = new_instance


class A(metaclass=SingleMeta):
    ...

class B(metaclass=SingleMeta):
    ...

a = A()
a1 =A()
print(a is a1)




# 使用 类装饰器


def single_class(cls):

    cls._instance = None
    origin_new = cls.__new__

    def new_(cls, *args, **kwargs):
        if cls._instance:
            return cls._instance
        cls._instance = cv = origin_new(cls)
        return cv

    cls.__new__ = new_

    return cls


@single_class
class A():
    ...


a = A()
b = A()

print(a is b )  # True



# 或者


def single_class(cls):
    cls._instance = None
    
    def new_(cls, *args, **kw):
        if cls._instance:
            return cls._instance
        cls._instance = cv = object.__new__(cls)
        return cv

    cls.__new__ = new_
    return cls



@single_class
class A(object):
    ...


a = A()
b = A()

print(a is b )

类装饰器， 扩充类的功能

比如重写 特殊方法 __getattribute__

某种程度上来讲，类装饰器方案就显得更加直观，并且它不会引入新的继承体系。 它的运行速度也更快一些， 因为他并不依赖 super() 函数

def log_getattribute(cls):
    # Get the original implementation
    orig_getattribute = cls.__getattribute__

    # Make a new definition
    def __getattribute__(cls, name):
        print('getting:', name)
        return orig_getattribute(cls, name)

    cls.__getattribute__ = __getattribute__


    # def new_getattribute(cls, name):
    #     print('getting:', name)
    #     return orig_getattribute(cls, name)
    # 
    # cls.__getattribute__ = new_getattribute


    # Attach to the class and return
    return cls

# Example use
@log_getattribute
class A:
    def __init__(self,x):
        self.x = x
    def spam(self):
        pass


a = A(42)
print(a.x)


# 类装饰器实现单例模式

def single_class(cls):

    cls._instance = None
    origin_new = cls.__new__

    def new_(cls, *args, **kwargs):
        if cls._instance:
            return cls._instance
        cls._instance = cv = origin_new(cls)
        return cv

    cls.__new__ = new_

    return cls


@single_class
class A():
    ...


a = A()
b = A()

print(a is b )  # True

或者使用元类

class LoggedGetattribute(object):
    def __getattribute__(self, name):
        print('getting:', name)
        return super().__getattribute__(name)

# Example:
class A(LoggedGetattribute):
    def __init__(self,x):
        self.x = x
    def spam(self):
        pass


a = A(42)
print(a.x)




class LoggedGetattribute(type):
    def __init__(cls, name, bases, dict):
        origin_getattr = cls.__getattribute__

        def getattr(self, *args, **kwargs):
            print('__getattribute__:', args, kwargs)
            return origin_getattr(self, *args, **kwargs)

        cls.__getattribute__ = getattr


class A(object, metaclass=LoggedGetattribute):  # Python 3 是 class A(object,metaclass=TraceAttribute):
    a = 1
    b = 2


a = A()
print(a.a)
# output: __getattribute__:('a',){}