单例模式

单例模式是一种设计模式一次只允许产生一个实例对象.

实现方法有很多,常见的方法有五种:

1.classmethod

class Mysql:
    _instance = None
    def __init__(self,host,port):
        self.host = host
        self.port = port
    @classmethod
    def singleton(cls):
        if not cls.singleton:
            _instance = cls('127.0.0.1',3306)
        return cls._instance

print(id(Mysql.singleton()))
print(id(Mysql.singleton()))

2.装饰器

from functools import wraps
def singleton(cls):
    cls._instance = cls('127.0.0.1',3306)
    @wraps(cls)
    def inner(*args,**kwargs):
        if args or kwargs:
            obj = cls(*args,**kwargs)
            return obj
        return cls._instance
    return inner


@singleton  # Mysql = singleton(Mysql)
class Mysql:
    def __init__(self,host,port):
        self.host = host
        self.port = port
obj1 = Mysql()
obj2 = Mysql()
print(id(obj1),id(obj2))

3.基于元类

class MymetaClass(type):
    def __call__(self,*args,**kwargs):
        if not getattr(self,'_instance'):
            self._instance = super().__call__(*args,**kwargs)
        return self._instance

class Mysql(metaclass = MymetaClass):
    _instance =None
    def __init__(self,host,port):
        self.host = host
        self.port = port
        
print(id(Mysql('127.9.0.1',9090)))
print(id(Mysql('127.9.0.2',9090)))

4基于__new__

class Singleton(object):

　　_instance = None

　　def __new__ (cls,*args,**kw):

　　　　if not cls._instance:

　　　　　　cls._instance = super(Singleton,cls).__new__(cls,*args,**kwargs)

　　　　　　return cls._instance

5基于模块(最常用!)

# demo.py
class Setting():
    pass

settings = Setting()


# start.py
from demo import settings

# settings被导入任何模块都是同一对象

以上五种方法,模块最简单也最常用.