单例模式

单例模式是一种软件设计模式，大概有四种方法来实现。

#  第一种方式，使用模块，
#  mysingleton.py


class My_Singleton(object):
    def foo(self):
        pass

my_singleton = My_Singleton()


#  第二种方式：使用__new__
class Singleton(object):
    _instance = None
    def __new__(cls,*args,**kwargs):
        if not cls._instance:
            cls._instance = super(Singleton,cls).__new__(cls,*args,**kwargs)
        return cls._instance

class MyClass(Singleton):
    a = 'adsgas'

"""
>>> one = MyClass()
>>> two = MyClass()
>>> one == two
True
>>> one is two
True
>>> id(one), id(two)
(4303862608, 4303862608)
"""

#  第三种方法：使用装饰器
from functools import wraps

def singleton(cls):
    instances = {}
    @wraps(cls)
    def getinstance(*args,**kwargs):
        if cls not in instances:
            instances[cls]= cls(*args,**kwargs)
        return instances[cls]

@singleton
class MyClass(object):
    a = 'dasdas'


#  使用metaclass,原类实现单例模式

class Singleton(type):
    _instances = {}
    def __call__(cls,*args,**kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton,cls).__call__(*args,**kwargs)
        return cls._instances[cls]
    
class MyClass(metaclass=Singleton):
    pass