方法一:裝飾器
利用“裝飾器只會執行一次”這個特點
def singleton(cls):
instances = []# 爲什麼這裏不直接爲None,因爲內部函數沒法訪問外部函數的非容器變量
def getinstance(*args, **kwargs):
if not instances:
instances.append(cls(*args, **kwargs))
return instances[0]
return getinstance
@singleton
class Foo:
a = 1
f1 = Foo()
f2 = Foo()
print id(f1), id(f2)
方法二:基類
利用“類變量對所有對象唯一”,即cls._instance
class Singleton(object):
def __new__(cls, *args, **kwargs):
if not hasattr(cls, '_instance'):
cls._instance = object.__new__(cls, *args, **kwargs)
return cls._instance
class Foo(Singleton):
a = 1
方法三:metaclass
利用“類變量對所有對象唯一”,即cls._instance
class Singleton(type):
def __call__(cls, *args, **kwargs):
if not hasattr(cls, '_instance'):
cls._instance = super(Singleton, cls).__call__(*args, **kwargs)
return cls._instance
class Foo():
__metaclass__ = Singleton
方法四:Borg模式
利用“類變量對所有對象唯一”,即__share_state
class Foo:
__share_state = {}
def __init__(self):
self.__dict__ = self.__share_state
方法五:利用import
利用“模塊只會被import一次”
#在文件mysingleton中
class Foo(object):
pass
f = Foo()
然後在其它模塊,from mysingleton import f
直接拿f當作單例的對象來用