单例模式

一. 单例模式四种方式

用于mysql连接时，防止每次操作都去创建一个数据库连接，使用单例模式仅在第一次创建连接，以后均使用该连接即可。

1. 文件导入

2. 基于类

无法支持多线程

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()

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()

3. 使用__new__

无法支持多线程

class Singleton(object):

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

    def __new__(cls, *args, **kwargs):

        if not hasattr(Singleton, "instance"):
            Singleton.instance = object.__new__(cls)
        return Singleton.instance


obj = Singleton("alex")
obj1 = Singleton("alex")
obj2 = Singleton("alex")

支持多线程

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)
                return Singleton.instance
        return Singleton.instance

def task():
    obj = Singleton()
    print(obj)

for i in range(5):
    t = threading.Thread(target=task)
    t.start()

4. 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)