在python中怎么实现单例模式
在python中怎么实现单例模式?下面给大家带来七种不同的方法:
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一:staticmethod
代码如下:
class Singleton(object): instance = None def __init__(self): raise SyntaxError('can not instance, please use get_instance') def get_instance(): if Singleton.instance is None: Singleton.instance = object.__new__(Singleton) return Singleton.instance a = Singleton.get_instance() b = Singleton.get_instance() print('a id=', id(a)) print('b id=', id(b))
该方法的要点是在__init__抛出异常,禁止通过类来实例化,只能通过静态get_instance函数来获取实例;因为不能通过类来实例化,所以静态get_instance函数中可以通过父类object.__new__来实例化。
二:classmethod
和方法一类似,代码:
class Singleton(object): instance = None def __init__(self): raise SyntaxError('can not instance, please use get_instance') def get_instance(cls): if Singleton.instance is None: Singleton.instance = object.__new__(Singleton) return Singleton.instance a = Singleton.get_instance() b = Singleton.get_instance() print('a id=', id(a)) print('b id=', id(b))
该方法的要点是在__init__抛出异常,禁止通过类来实例化,只能通过静态get_instance函数来获取实例;因为不能通过类来实例化,所以静态get_instance函数中可以通过父类object.__new__来实例化。
三:类属性方法
和方法一类似, 代码:
class Singleton(object): instance = None def __init__(self): raise SyntaxError('can not instance, please use get_instance') def get_instance(): if Singleton.instance is None: Singleton.instance = object.__new__(Singleton) return Singleton.instance a = Singleton.get_instance() b = Singleton.get_instance() print(id(a)) print(id(b))
该方法的要点是在__init__抛出异常,禁止通过类来实例化,只能通过静态get_instance函数来获取实例;因为不能通过类来实例化,所以静态get_instance函数中可以通过父类object.__new__来实例化。
四:__new__
常见的方法, 代码如下:
class Singleton(object): instance = None def __new__(cls, *args, **kw): if not cls.instance: # cls.instance = object.__new__(cls, *args) cls.instance = super(Singleton, cls).__new__(cls, *args, **kw) return cls.instance a = Singleton() b = Singleton() print(id(a)) print(id(b))
五:装饰器
代码如下:
def Singleton(cls): instances = {} def getinstance(): if cls not in instances: instances[cls] = cls() return instances[cls] return getinstance class MyClass: pass a = MyClass() b = MyClass() c = MyClass() print(id(a)) print(id(b)) print(id(c))
六:元类
python2版:
class Singleton(type): def __init__(cls, name, bases, dct): super(Singleton, cls).__init__(name, bases, dct) cls.instance = None def __call__(cls, *args): if cls.instance is None: cls.instance = super(Singleton, cls).__call__(*args) return cls.instance class MyClass(object): __metaclass__ = Singleton a = MyClass() b = MyClass() c = MyClass() print(id(a)) print(id(b)) print(id(c)) print(a is b) print(a is c)
或者:
class Singleton(type): def __new__(cls, name, bases, attrs): attrs["_instance"] = None return super(Singleton, cls).__new__(cls, name, bases, attrs) def __call__(cls, *args, **kwargs): if cls._instance is None: cls._instance = super(Singleton, cls).__call__(*args, **kwargs) return cls._instance class Foo(object): __metaclass__ = Singleton x = Foo() y = Foo() print(id(x)) print(id(y))
python3版:
class Singleton(type): def __new__(cls, name, bases, attrs): attrs['instance'] = None return super(Singleton, cls).__new__(cls, name, bases, attrs) def __call__(cls, *args, **kwargs): if cls.instance is None: cls.instance = super(Singleton, cls).__call__(*args, **kwargs) return cls.instance class Foo(metaclass=Singleton): pass x = Foo() y = Foo() print(id(x)) print(id(y))
七:名字覆盖
代码如下:
class Singleton(object): def foo(self): print('foo') def __call__(self): return self Singleton = Singleton() Singleton.foo() a = Singleton() b = Singleton() print(id(a)) print(id(b))
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