python中7種實現單例模式的方法:staticmethod、classmethod、類屬性方法、__new__、裝飾器、元類、名字覆蓋
阿新 • • 發佈:2019-02-03
本文的以下實現方法為了簡單起見不考慮執行緒安全。
一:staticmethod
程式碼如下:class Singleton(object): instance = None def __init__(self): raise SyntaxError('can not instance, please use get_instance') @staticmethod def get_instance(): if Singleton.instance is None: Singleton.instance = object該方法的要點是在__init__丟擲異常,禁止通過類來例項化,只能通過靜態get_instance函式來獲取例項;因為不能通過類來例項化,所以靜態get_instance函式中可以通過父類object.__new__來例項化。.__new__(Singleton) return Singleton.instance a = Singleton.get_instance() b = Singleton.get_instance() print('a id=', id(a)) print('b id=', id(b))
二:classmethod
和方法一類似,程式碼:class該方法的要點是在__init__丟擲異常,禁止通過類來例項化,只能通過靜態get_instance函式來獲取例項;因為不能通過類來例項化,所以靜態get_instance函式中可以通過父類object.__new__來例項化。Singleton(object): instance = None def __init__(self): raise SyntaxError('can not instance, please use get_instance') @classmethod 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))
三:類屬性方法
和方法一類似, 程式碼: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))該方法的要點是在__init__丟擲異常,禁止通過類來例項化,只能通過靜態get_instance函式來獲取例項;因為不能通過類來例項化,所以靜態get_instance函式中可以通過父類object.__new__來例項化。
四:__new__
常見的方法, 程式碼如下:
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:裝飾器
程式碼如下:
def Singleton(cls): instances = {} def getinstance(): if cls not in instances: instances[cls] = cls() return instances[cls] return getinstance @Singleton class MyClass: pass a = MyClass() b = MyClass() c = MyClass() print(id(a)) print(id(b)) print(id(c))
六:元類
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:名字覆蓋
程式碼如下: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))