单例模式

1、文件引入是一种单例模式

2、自定义类方法（有代价，告知所有人，以后实例化时，不要再 类(),使用 类.instance() ），无法支持多线程 50分

			class Foo(object):
				_instance = None

				def __init__(self,name):
					self.name = name

				@classmethod
				def instance(cls,*args,**kwargs):
					if hasattr(cls,'_instance'):
						obj = cls(*args,**kwargs)
						setattr(cls,'_instance,obj)
					return cls._instance


			obj1 = Foo.instance('alex')
			obj2 = Foo.instance('alex')
			print(id(obj1),id(obj2))

　　

3、自定义类方法（支持多线程）70分

import time
import threading
class Singleton(object):
    _instance_lock = threading.Lock()

    def __init__(self):
        time.sleep(1)

    @classmethod
    def instance(cls, *args, **kwargs):
        if not hasattr(Singleton, "_instance"):
            with Singleton._instance_lock:
                if not hasattr(Singleton, "_instance"):
                    Singleton._instance = Singleton(*args, **kwargs)
        return Singleton._instance


def task(arg):
    obj = Singleton.instance()
    print(obj)
for i in range(10):
    t = threading.Thread(target=task,args=[i,])
    t.start()
time.sleep(20)
obj = Singleton.instance()
print(obj)

4、基于__new__方法实现单例模式 80分（对于用户更友好，和普通实例化一样）

# class Singleton(object):
#     def __init__(self):
#         print('init',self)
#
#
#     def __new__(cls, *args, **kwargs):
#         o = object.__new__(cls, *args, **kwargs)
#         print('new',o)
#         return o
#
# obj = Singleton()
#
# print('xxx',obj)

#先执行__new__方法，创建对象，再执行__init__方法，再赋值给obj，他们三个打印的对象是一样的。

import time
import threading
class Singleton(object):
    _instance_lock = threading.Lock()

    def __init__(self):
        pass


    def __new__(cls, *args, **kwargs):
        if not hasattr(Singleton, "_instance"):
            with Singleton._instance_lock:
                if not hasattr(Singleton, "_instance"):
                    Singleton._instance = object.__new__(cls, *args, **kwargs)
        return Singleton._instance

obj1 = Singleton()
obj2 = Singleton()
print(obj1,obj2)

 5、基于metaclass方式实现

"""
1.对象是类创建，创建对象时候类的__init__方法自动执行，对象()执行类的 __call__ 方法
2.类是type创建，创建类时候type的__init__方法自动执行，类() 执行type的 __call__方法(类的__new__方法,类的__init__方法)

# 第0步: 执行type的 __init__ 方法【类是type的对象】
class Foo:
    def __init__(self):
        pass

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

# 第1步: 执行type的 __call__ 方法
#        1.1  调用 Foo类（是type的对象）的 __new__方法，用于创建对象。
#        1.2  调用 Foo类（是type的对象）的 __init__方法，用于对对象初始化。
obj = Foo()
# 第2步：执行Foo的 __call__ 方法
obj()
"""


"""
class SingletonType(type):
    def __init__(self,*args,**kwargs):
        super(SingletonType,self).__init__(*args,**kwargs)

    def __call__(cls, *args, **kwargs):
        obj = cls.__new__(cls,*args, **kwargs)
        cls.__init__(obj,*args, **kwargs) # Foo.__init__(obj)
        return obj

class Foo(metaclass=SingletonType):
    def __init__(self,name):
        self.name = name
    def __new__(cls, *args, **kwargs):
        return object.__new__(cls, *args, **kwargs)

obj = Foo('name')
"""

执行顺序：

import threading

class SingletonType(type):
    _instance_lock = threading.Lock()
    def __call__(cls, *args, **kwargs):
        if not hasattr(cls, "_instance"):
            with SingletonType._instance_lock:
                if not hasattr(cls, "_instance"):
                    cls._instance = super(SingletonType,cls).__call__(*args, **kwargs)
        return cls._instance

class Foo(metaclass=SingletonType):
    def __init__(self,name):
        self.name = name


obj1 = Foo('name')
obj2 = Foo('name')
print(obj1,obj2)