单例模式的几种方法，以及应用

一、类方法

1.此方法不支持多线程

import time
import threading
class Singleton():
    def __init__(self):
        time.sleep(1)

    @classmethod
    def instance(cls,*args,**kwargs):
        if not hasattr(Singleton,"_instance"):
            Singleton._instance = Singleton(*args,**kwargs)
        return Singleton._instance
# obj = Singleton.instance()
# obj2 = Singleton.instance()
def task(arg):
    obj = Singleton.instance()
    print(obj)
if __name__ == '__main__':
    for i in range(10):
        t = threading.Thread(target=task,args=(i,))
        t.start()

2.加锁，支持多线程

import time
import threading
class Singleton():
    _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
# obj = Singleton.instance()
# obj2 = Singleton.instance()
def task(arg):
    obj = Singleton.instance()
    print(obj)
if __name__ == '__main__':
    for i in range(10):
        t = threading.Thread(target=task,args=(i,))
        t.start()
time.sleep(10)
obj = Singleton.instance()
print(obj)

二、__new__方法

import time
import threading


class Singleton():
    _instance_lock = threading.Lock()

    def __init__(self):
        time.sleep(1)
        print('init', self)

    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


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


if __name__ == '__main__':
    for i in range(10):
        t = threading.Thread(target=task, args=(i,))
        t.start()

三、基于@metaclass创建单例模式
1.type与类间的关系

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Foodef __call__ 方法
obj()

2.执行顺序

class SinletonType(type):
    def __init__(self,*args,**kwargs):
        print('type.init',self)
        super(SinletonType,self).__init__(*args,**kwargs)
    def __call__(self, *args, **kwargs):
        print('type.call',self)
        obj = self.__new__(self,*args,**kwargs)
        self.__init__(obj,*args,**kwargs)
        return obj
class Foo(metaclass=SinletonType):
    def __init__(self):
        print('Foo.init')
    def __new__(cls, *args, **kwargs):
        print('Foo.new')
        return object.__new__(cls,*args,**kwargs)
obj = Foo()
'''
type.init <class '__main__.Foo'>
type.call <class '__main__.Foo'>
Foo.new
Foo.init
'''

3.单例

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)

四、装饰器

import threading


def Singleton(cls):
    _instance = {}
    def _singleton(*args, **kargs):
        if cls not in _instance:
            with threading.Lock():
                if cls not in _instance:
                    _instance[cls] = cls(*args, **kargs)
        return _instance[cls]

    return _singleton


@Singleton
class A(object):
    a = 1

    def __init__(self, x=0):
        self.x = x


a1 = A(2)
a2 = A(3)
print(
    id(a1))
print(
    id(a2))
print(
    a1.x)
print(
    a2.x)

五、应用

数据库连接池+单例模式

############pool.py#######################
import pymysql
import threading
from DBUtils.PooledDB import PooledDB

class SingletonDBPool(object):
    _instance_lock = threading.Lock()

    def __init__(self):
        self.pool = PooledDB(
            creator=pymysql,  # 使用链接数据库的模块
            maxconnections=6,  # 连接池允许的最大连接数，0和None表示不限制连接数
            mincached=2,  # 初始化时，链接池中至少创建的空闲的链接，0表示不创建

            maxcached=5,  # 链接池中最多闲置的链接，0和None不限制
            maxshared=3,
            # 链接池中最多共享的链接数量，0和None表示全部共享。PS: 无用，因为pymysql和MySQLdb等模块的 threadsafety都为1，所有值无论设置为多少，_maxcached永远为0，所以永远是所有链接都共享。
            blocking=True,  # 连接池中如果没有可用连接后，是否阻塞等待。True，等待；False，不等待然后报错
            maxusage=None,  # 一个链接最多被重复使用的次数，None表示无限制
            setsession=[],  # 开始会话前执行的命令列表。如：["set datestyle to ...", "set time zone ..."]
            ping=0,
            # ping MySQL服务端，检查是否服务可用。# 如：0 = None = never, 1 = default = whenever it is requested, 2 = when a cursor is created, 4 = when a query is executed, 7 = always
            host='127.0.0.1',
            port=3306,
            user='root',
            password='123',
            database='pooldb',
            charset='utf8'
        )

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

    def connect(self):
        return self.pool.connection()

#app.py
from pool import SingletonDBPool

def run():
    pool = SingletonDBPool()
    con = pool.connect()
    # xxxxxx

    con.close()

if __name__ == '__main__':
    run()