python实现的单子模式（附解释）

lygyh9985825

• 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