Python 中单例模式 4种方法

假设我们在一个与数据库交互的系统中，需要进行数据库操作，那么我们就有必要了解单例模式，单例模式使得我们不用每次进行数据库的操作时都去链接数据库。

我将循环渐进，由浅入深的写一下单例模式示例。

实例1：

db.py

class Foo(object):
    def __init__(self):
        self.conn = "连接数据库"

    def get(self):
        return self.conn

obj = Foo()

views.py

import db

print(db.obj)

run.py

import db
import views

print(db.obj)

执行run.py,打印结果如下，这就是单例模式

<db.Foo object at 0x000001C1D8FC5A58>
<db.Foo object at 0x000001C1D8FC5A58>

实例二：

注意这里使用了锁来解决线程安全问题

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()

实例三：

import threading

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

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

实例四：

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)