单例模式

基于文件导入实现单例模式：

　db.py：

class Foo(object):

    def __init__(self):
        self.conn = "连接数据库"
        pass

    def get(self):
        self.conn
obj = Foo()

import redis
obj = redis.Redis(host='10.211.55.4', port=6379)

　views.py：

import db


print(db.obj)

　run.py：

import db
import views

print(db.obj)

基于 classmethod ：　

import threading
import time

class Foo(object):
    instance = None
    lock = threading.Lock()

    def __init__(self):
        self.a1 = 1
        self.a2 = 2
        import time
        import random
        time.sleep(2)

    @classmethod
    def get_instance(cls,*args,**kwargs):
        if not cls.instance:
            with cls.lock:
                if not cls.instance:
                    obj = cls(*args,**kwargs)
                    cls.instance = obj
                return cls.instance
        return cls.instance
def task():
    obj = Foo.get_instance()
    print(obj)

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

time.sleep(10)
Foo.get_instance()

基于__new__：

import threading
import time

class Foo(object):
    instance = None
    lock = threading.Lock()

    def __init__(self):
        self.a1 = 1
        self.a2 = 2
        import time
        import random
        time.sleep(2)

    def __new__(cls, *args, **kwargs):
        if not cls.instance:
            with cls.lock:
                if not cls.instance:
                    obj = super(Foo,cls).__new__(cls,*args, **kwargs)
                    cls.instance = obj
                return cls.instance
        return cls.instance


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

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

基于metaclass：

import threading

lock = threading.Lock()

class Singleton(type):
    def __call__(cls, *args, **kwargs):
        if not hasattr(cls, 'instance'):
            with lock:
                if not hasattr(cls,'instance'):
                    obj = cls.__new__(cls,*args, **kwargs)
                    obj.__init__(*args, **kwargs)
                    setattr(cls,'instance',obj)
                return getattr(cls,'instance')
        return getattr(cls, 'instance')


class Foo(object,metaclass=Singleton):
    def __init__(self):
        self.name = 'alex'

obj1 = Foo()
obj2 = Foo()

print(obj1,obj2)

详细信息：https://www.cnblogs.com/huchong/p/8244279.html