python基础之单例模式

class Myclass(object):
    def foo(self):
        print(‘Myclass.foo‘)
my_class_obj=Myclass()

class Myclass(object):
    _instance =None
    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance=super(Myclass, cls).__new__(cls,*args,**kwargs)
        return cls._instance
#我们将类的实例和一个类变量_instance关联起来,如果cls._instance为None则创建实例,否者直接返回cls._instance
class Herclass(Myclass):
    a=1

one=Herclass()
two=Herclass()
print(one==two)
print(one is two)
print(id(one),id(two))

from functools import wraps

def sin(cls):
    instances={}

    @wraps(cls)
    def get(*args,**kwargs):
        if cls not in instances:
            instances[cls] =cls(*args,**kwargs)
        return instances[cls]

@sin
class Myclass(object):
    a =1

#我们定义了一个装饰器sin,他返回了一个内部函数get,该函数会判断某一个类是否在字典instance中,如果不存在,就会将cls
#作为key,cls(*args, **kw) 作为 value 存到 instances 中，否则，直接返回 instances[cls]。

import threading


class Singleton2(type):
    def __init__(cls, name, bases, dict):
        super(Singleton2, cls).__init__(name, bases, dict)
        cls._instance = None

    def __call__(cls, *args, **kw):
        if cls._instance is None:
            cls._instance = super(Singleton2, cls).__call__(*args, **kw)
        return cls._instance

#python2
class MyClass(object):
    __metaclass__ = Singleton2


one = MyClass()
two = MyClass()
two.a = 3

#print one.a
# 3
#print id(one)
# 31495472
#print id(two)
# 31495472