SingletonProgramming (original) (raw)

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Singleton

See also http://c2.com/cgi/wiki?PythonSingleton and http://www.python.org/workshops/1997-10/proceedings/savikko.html

classmethod

pro

• You can use both as simple class or as a singleton.
• You do not need to write code for each class you want to act as singleton.

cons

1 2 class Singleton: 3 _singleton = None 4 def getSingleton(cls): 5 if not isinstance(cls._singleton,cls): 6 cls._singleton = cls() 7 return cls._singleton 8 9 getSingleton = classmethod(getSingleton) 10 11 if name=='main': 12 class Test(Singleton): 13 def test(self): 14 print self.class,id(self) 15 16 class Test1(Test): 17 def test1(self): 18 print self.class,id(self),'Test1' 19 20 t1 = Test.getSingleton() 21 t2 = Test.getSingleton() 22
23 t1.test() 24 t2.test() 25 assert(isinstance(t1,Test)) 26 assert(isinstance(t2,Test)) 27 assert(id(t1)==id(t2)) 28 29 t1 = Test1.getSingleton() 30 t2 = Test1.getSingleton() 31 32 assert(isinstance(t1,Test1)) 33 assert(isinstance(t2,Test1)) 34 assert(id(t1)==id(t2)) 35
36 t1.test() 37 t1.test1() 38 t2.test() 39 t1.test1() 40 41 t3 = Test.getSingleton() 42
43 t3.test() 44 assert(isinstance(t3,Test))