- 一、通过metaclass来实现
class Singleton(type): def __init__(self, name, bases, dct): super(Singleton, self).__init__(name, bases, dct) self.instance = None def __call__(self,*args,**kw): if self.instance is None: self.instance = super(Singleton, self).__call__(*args, **kw) return self.instance class A(object): __metaclass__ = Singleton def __init__(self): self.xx = 1 A()
注解:
__metaclass__顾名思义是class A的元类,所谓元类即其实例是一个类。所以,Singleton的一个实例是class A。那么A()到底是怎样的执行过程呢?
- 1) Singleton实例化:隐式调用了Sinleton的__new__方法生成一个实例,并调用Singleton.__init__去初始化这个实例,初始化之后便产生了class A。这里,name, bases, dct就是class A的定义中的相关属性。
- 2) A的实例化,由于在Singleton中定义了__call__,即Singleton的实例可以像函数一样调用。然而,Singleton的实例是class A,根据规则,在实例化A时即A(),解释器选择调用Singleton.__call__(并不调用A.__new__和A.__init__)。那么A.__init__在何时调用呢?
- 3) 在执行Singleton.__Call__时,如果self.instance(即A.instance)是None时,则调用super(Singleton, self).__call__,在这个函数中会调用A.__init__,初始化A的实例后存在self.instance(即A.instance)中。
通过这样就可以做到只有一个A的实例,并存储在A.instance中。元类确实有点绕人,但是弄清楚了之后,它在特定的时刻能带来很大的方便。
- 二、通过decorator来实现
class Singleton: """ A non-thread-safe helper class to ease implementing singletons. This should be used as a decorator -- not a metaclass -- to the class that should be a singleton. The decorated class can define one `__init__` function that takes only the `self` argument. Other than that, there are no restrictions that apply to the decorated class. To get the singleton instance, use the `Instance` method. Trying to use `__call__` will result in a `TypeError` being raised. Limitations: The decorated class cannot be inherited from. """ def __init__(self, decorated): self._decorated = decorated def Instance(self): """ Returns the singleton instance. Upon its first call, it creates a new instance of the decorated class and calls its `__init__` method. On all subsequent calls, the already created instance is returned. """ try: return self._instance except AttributeError: self._instance = self._decorated() return self._instance def __call__(self): raise TypeError('Singletons must be accessed through `Instance()`.') def __instancecheck__(self, inst): return isinstance(inst, self._decorated)
@Singleton class Foo: def __init__(self): print 'Foo created' f = Foo() # Error, this isn't how you get the instance of a singleton f = Foo.Instance() # Good. Being explicit is in line with the Python Zen g = Foo.Instance() # Returns already created instance print f is g # True
- 三、通过__new__来实现
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 if __name__ == '__main__': s1=Singleton() s2=Singleton() if(id(s1)==id(s2)): print "Same" else: print "Different"
部分源码来自于: http://stackoverflow.com/questions/42558/python-and-the-singleton-pattern
http://stackoverflow.com/questions/6760685/creating-a-singleton-in-python