Flask中的单例模式

时间：2023-03-09 16:00:29
Flask中的单例模式

1,基于文件的单例模式：

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='123456',

            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()

from pool import SingletonDBPool

def run():

    pool = SingletonDBPool()

    con = pool.connect()

    # xxxxxx

    con.close()

if __name__ == '__main__':

    run()

　　

　　

# 单例模式：无法支持多线程情况

"""

class Singleton(object):

    def __init__(self):

        import time

        time.sleep(1)

    @classmethod

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

        if not hasattr(Singleton, "_instance"):

            Singleton._instance = Singleton(*args, **kwargs)

        return Singleton._instance

import threading

def task(arg):

    obj = Singleton.instance()

    print(obj)

for i in range(10):

    t = threading.Thread(target=task,args=[i,])

    t.start()

"""

# # 单例模式：支持多线程情况

"""

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)

"""

　

基于__new__方法实现单例模式

# 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)

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)

# def task(arg):

#     obj = Singleton()

#     print(obj)

#

# for i in range(10):

#     t = threading.Thread(target=task,args=[i,])

#     t.start()

　　

基于metaclass

"""

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()

"""

"""

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')

"""

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)

　

# ######################## 基于 类方法实现 #########################

"""

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

# 使用先说明，以后用单例模式，obj = Singleton.instance()

# 示例：

# obj1 = Singleton.instance()

# obj2 = Singleton.instance()

# print(obj1,obj2)

# 错误示例

# obj1 = Singleton()

# obj2 = Singleton()

# print(obj1,obj2)

"""

# ######################### 基于__new__方式实现 #########################

"""

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

# 使用先说明，以后用单例模式，obj = Singleton()

# 示例

# obj1 = Singleton()

# obj2 = Singleton()

# print(obj1,obj2)

"""

# ######################### 基于metaclass方式实现 ###################

"""

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)

"""

# PS: 为了保证线程安全在内部加入锁
# 装饰器实现的单例模式

#

def wrapper(cls):

    instance = {}

    def inner(*args,**kwargs):

        if cls not in  instance:

            instance[cls] = cls(*args,**kwargs)

        return instance[cls]

    return inner

@wrapper

class Singleton(object):

    def __init__(self,name,age):

        self.name = name

        self.age = age

obj1 = Singleton('haiyan',22)

obj2 = Singleton('xx',22)

print(obj1)

print(obj2)

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