单例模式是常见的一种设计模式，它是针对类的一种描述，因此，我们可以使用python的decorator来实现通用的单例模式。

一.基本的单例模式

首先建立我们的decorator。我们需要为classType建立_instance和_lock成员：

Python

def singleton(classType):

classType._instance = None

classType._lock = Lock()

return classType

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defsingleton(classType):

classType._instance=None

classType._lock=Lock()

returnclassType

然后为class建立getInstance函数：

Python

def __getInstance(classname):

if classname._instance is None:

classname._lock.acquire()

if classname._instance is None:

classname._instance = classname()

classname._lock.release()

return classname._instance

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def__getInstance(classname):

ifclassname._instanceisNone:

classname._lock.acquire()

ifclassname._instanceisNone:

classname._instance=classname()

classname._lock.release()

returnclassname._instance

然后在我们的decorator中，将getInstance函数添加到class上：

Python

classType.getInstance = classmethod(__getInstance)

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classType.getInstance=classmethod(__getInstance)

现在我们的代码长这样。这个decorator已经具备了基本功能：

Python

def __getInstance(classType):

if classType._instance is None:

classType._lock.acquire()

if classType._instance is None:

classType._instance = classType()

classType._lock.release()

return classType._instance

def singleton(classType):

classType._instance = None

classType._lock = Lock()

classType.getInstance = classmethod(__getInstance)

return classType

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def__getInstance(classType):

ifclassType._instanceisNone:

classType._lock.acquire()

ifclassType._instanceisNone:

classType._instance=classType()

classType._lock.release()

returnclassType._instance

defsingleton(classType):

classType._instance=None

classType._lock=Lock()

classType.getInstance=classmethod(__getInstance)

returnclassType

使用方法如下：

Python

@singleton

class XXX(object):

# add class definitions

pass

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@singleton

classXXX(object):

# add class definitions

pass

二.可继承的单例模式

现在让我们为这个单例模式添加一些功能：我们让被这个单例类可以产生子类，且子类也是单例类，和父类共享同一个instance。为此，我们必须修改被修饰的单例类的init方法，在其中设置instance为自身：

Python

def __singleton_init(self):

# check whether instance has existed

if self._instance is not None:

raise SingletonExistedError(type(self))

# set instance of this class and its ancestor classes to self

for baseClass in inspect.getmro(type(self)):

if '_instance' in baseClass.__dict__:

baseClass._instance = self

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def__singleton_init(self):

# check whether instance has existed

ifself._instanceisnotNone:

raiseSingletonExistedError(type(self))

# set instance of this class and its ancestor classes to self

forbaseClassininspect.getmro(type(self)):

if'_instance'inbaseClass.__dict__:

baseClass._instance=self

SingletonExistedError为自定义的error类型：

Python

class SingletonExistedError(Exception):

def __init__(self, classType):

super(SingletonExistedError, self).__init__('trying to construct instance of singleton class ' + classType.__name__ + ' while an instance has already existed!')

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classSingletonExistedError(Exception):

def__init__(self,classType):

super(SingletonExistedError,self).__init__('trying to construct instance of singleton class '+classType.__name__+' while an instance has already existed!')

现在我们需要把这个新的init函数添加到被修饰的单例类的init函数上。很自然的，我们又可以利用函数的decorator来办成这件事情，就像这样：

Python

def __singleton_init_decorator(init_func):

def __singleton_init(self):

# check whether instance has existed

if self._instance is not None:

raise SingletonExistedError(type(self))

# set instance of the class and ancestor singletons to self

for baseClass in inspect.getmro(type(self)):

if '_instance' in baseClass.__dict__:

baseClass._instance = self

init_func(self)

return __singleton_init

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def__singleton_init_decorator(init_func):

def__singleton_init(self):

# check whether instance has existed

ifself._instanceisnotNone:

raiseSingletonExistedError(type(self))

# set instance of the class and ancestor singletons to self

forbaseClassininspect.getmro(type(self)):

if'_instance'inbaseClass.__dict__:

baseClass._instance=self

init_func(self)

return__singleton_init

最后，在我们的singleton decorator上加上这句代码：

Python

classType.__init__ = __singleton_init_decorator(classType.__init__)

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classType.__init__=__singleton_init_decorator(classType.__init__)

大功告成！附上完整代码：

Python

class SingletonExistedError(Exception):

def __init__(self, classType):

super(SingletonExistedError, self).__init__('trying to construct instance of singleton class ' + classType.__name__ + ' while an instance has already existed!')

def __getInstance(classType):

if classType._instance is None:

classType._lock.acquire()

if classType._instance is None:

classType._instance = classType()

classType._lock.release()

return classType._instance

def __singleton_init_decorator(init_func):

def __singleton_init(self):

# check whether instance has existed

if self._instance is not None:

raise SingletonExistedError(type(self))

# set instance of the class and ancestor singletons to self

for baseClass in inspect.getmro(type(self)):

if '_instance' in baseClass.__dict__:

baseClass._instance = self

init_func(self)

return __singleton_init

def singleton(classType):

classType._instance = None

classType._lock = Lock()

classType.getInstance = classmethod(__getInstance)

classType.__init__ = __singleton_init_decorator(classType.__init__)

return classType

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classSingletonExistedError(Exception):

def__init__(self,classType):

super(SingletonExistedError,self).__init__('trying to construct instance of singleton class '+classType.__name__+' while an instance has already existed!')

def__getInstance(classType):

ifclassType._instanceisNone:

classType._lock.acquire()

ifclassType._instanceisNone:

classType._instance=classType()

classType._lock.release()

returnclassType._instance

def__singleton_init_decorator(init_func):

def__singleton_init(self):

# check whether instance has existed

ifself._instanceisnotNone:

raiseSingletonExistedError(type(self))

# set instance of the class and ancestor singletons to self

forbaseClassininspect.getmro(type(self)):

if'_instance'inbaseClass.__dict__:

baseClass._instance=self

init_func(self)

return__singleton_init

defsingleton(classType):

classType._instance=None

classType._lock=Lock()

classType.getInstance=classmethod(__getInstance)

classType.__init__=__singleton_init_decorator(classType.__init__)

returnclassType