python実装の単一サブモード(説明付)
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()
注記:
__metaclass__名前の通りclass Aのメタクラスであり、メタクラスとはその例がクラスである.したがって、Singletonの一例はclass Aである.ではA()はいったいどのような実行過程なのでしょうか.
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
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"
ソースコードの一部は次のとおりです.
http://stackoverflow.com/questions/42558/python-and-the-singleton-pattern
http://stackoverflow.com/questions/6760685/creating-a-singleton-in-python