#pragma section-numbers off

= 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 '''



{{{
#!python
# Code is Public Domain.
class Singleton:
    _singleton = None
    def getSingleton(cls):
        if not isinstance(cls._singleton,cls):
            cls._singleton = cls()
        return cls._singleton

    getSingleton = classmethod(getSingleton)

if __name__=='__main__':
    class Test(Singleton):
        def test(self):
            print self.__class__,id(self)

    class Test1(Test):
        def test1(self):
            print self.__class__,id(self),'Test1'

    t1 = Test.getSingleton()
    t2 = Test.getSingleton()
    
    t1.test()
    t2.test()
    assert(isinstance(t1,Test))
    assert(isinstance(t2,Test))
    assert(id(t1)==id(t2))

    t1 = Test1.getSingleton()
    t2 = Test1.getSingleton()

    assert(isinstance(t1,Test1))
    assert(isinstance(t2,Test1))
    assert(id(t1)==id(t2))
    
    t1.test()
    t1.test1()
    t2.test()
    t1.test1()

    t3 = Test.getSingleton()
    
    t3.test()
    assert(isinstance(t3,Test))
}}}