单例模式

单例模式总共分为四种：

第一种：文件导入

第二种：基于类分为可以支持多线程和不能支持多线程

（1）不能支持多线程　　

　　

import threading

class Singleton(object): 
    def __init__(self):
        pass
    @classmethod 
    def instance(cls, *args, **kwargs): 
        if not hasattr(Singleton, "_instance"): 
            Singleton._instance = Singleton(*args, **kwargs) 
    return Singleton._instance
    
obj = Singleton.instance()

（2）可以支持多线程

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


# 第一次调用
def task(arg):
    obj = Singleton.instance()
    print(obj)
for i in range(10):
    t = threading.Thread(target=task,args=[i,])
    t.start()


# 第二次调用    
time.sleep(20)
obj = Singleton.instance()　　

第三种：使用__new__方法

（1）不能支持多线程

class Singleton(object):
    
    Singleton._instance = None

    def __init__(self):
        pass


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

（2）可以支持多线程

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

第四种：基于metaclass

-------------------------------(一)-------------------------------

# 创建对象

class SingletonType(type):

    def __call__(cls, *args, **kwargs):
        obj = super(SingletonType,cls).__call__(*args, **kwargs)   #type类帮创建__new__和__init__并返回
        return obj

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

obj = Foo("alex")

-------------------------------(二)-------------------------------

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)