python singleton design pattern

decorate
baseclass
metaclass
import module
super()

一、A decorator

def singleton(class_):instances = {}def getinstance(*args, **kwargs):if class_ not in instances:instances[class_] = class_(*args, **kwargs)return instances[class_]return getinstance@singleton
class MyClass(BaseClass):pass

当用MyClass() 去创建一个对象时这个对象将会是单例的。MyClass 本身已经是一个函数。不是一个类，所以你不能通过它来调用类的方法。所以对于

m=MyClass() n = MyClass() o=type(n)() m==n and m!=o and n != o 将会是True

二、baseclass

class Singleton(object):_instance = Nonedef __new__(class_, *args, **kwargs):if not isinstance(class_._instance, class_):# class_._instance = object.__new__(class_)   这行语句和下一行语句作用一样的class_._instance=super(Singleton,class_).__new__(class_)return class_._instance
class MyClass(Singleton):def __init__(self,name):self.name = nameprint(name)

pros

　　是真的类

cons:

在多继承的时候要注意

三、metaclass

class Singleton(type):_instances = {}def __call__(cls, *args, **kwargs):if cls not in cls._instances:cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)return cls._instances[cls]#Python2
class MyClass(BaseClass):__metaclass__ = Singleton#Python3
class MyClass(BaseClass, metaclass=Singleton):pass

Pros

It's a true class
Auto-magically covers inheritance
Uses __metaclass__ for its proper purpose (and made me aware of it)

四、通过导入模块

五、

super(type[,object or type])

If the second argument is omitted, the super object returned is unbound. If the second argument is an object, isinstance(obj, type) must be true.

If the second argument is a type, issubclass(type2, type) must be true (this is useful for classmethods).

note :super() 只能用于新式类

链接 https://rhettinger.wordpress.com/2011/05/26/super-considered-super/

多继承，在python3 中全部都是新式类

新式类的继承顺序是广度优先，python2 中的经典类是深度优先

通过一个例子来理解

class A(object):def f1(self):print('a.f1')class B(A):def f2(self):print('b.f1')class F(object):def f1(self):print('f.f1')class C(B,F):def f3(self):print('c.f1')insta = C()
insta.f1()

关系图

将会打印出a.f1

如果代码改为

class A(object):def f1(self):print('a.f1')class B(A):def f2(self):print('b.f1')class F(A):def f1(self):print('f.f1')class C(B,F):def f3(self):print('c.f1')insta = C()
insta.f1()

关系图如下：

运行结果是f.f1

python 2 代码如下

class A:  #经典类    def foo(self):        print'A'class B(A):

    def foo(self):        print'B'class C(A):    pass    #def foo(self):     #   print'C'class D(B):    #def foo(self):     #   print 'D'    pass

class F(B):    #pass    def foo(self):        print 'F'        pass

class G(D,F):    passg1=G()g1.foo()  #打印出  B

python 3 代码

class A(object):def f1(self):print('a.f1')class B(A):pass def f1(self):print('b.f1')class C(A):def f1(self):print('c.f1')class D(B):pass# def f1(self):#     print('D.f1')class F(B):passdef f1(self):print('f.f1')class G(D,F):# def f1(self):#     print('g.f1')passinsta = G()
insta.f1()  #打印出f.f1

转载于:https://www.cnblogs.com/yuyang26/p/7717571.html