Webware 0.7/Singleton pattern

[pixie888 at hotmail.com]

Read the following links for a singletondiscussion in Python

http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/66531
http://www.aleax.it/Python/5ep.html

Henk

On 16 Jul 2002 00:45:41 -0700, mbub at meta-level.de (Michael Bub) wrote:

>Hi folks,
>
>I am currenty working on my diploma thesis. I chose to use Webware and
>Python to build my WebApp.
>
>During the development it turned out that some classes should become
>singletons.
>I implemented all my singleton classes with the following code:
>
>class myClass:
>
>    __instance=None
>
>    def __init__(self):
>       # init stuff
>       pass
>
>    def get_instance():
>       if myClass.__instance is None:
>           myClass.__instance=myClass()
>       return myClass.__instance
>    get_instance=staticmethod(get_instance)
>
>This generally seems to work just fine when using it. However, in my
>WebApp built with Webware 0.7 (using Python 2.2 on a FreeBSD4.4
>system), the second or third time the get_instance method is called,
>the one instance that was created by the singleton seems to be
>forgotten about (or rather, it seems as if the file that holds the
>class definition was imported and used for the very first time) and
>another one is created. This, of course, sucks. I do not know where
>the first one goes, its __del__ does not get called during that
>process, but it is replaced by the second instance created by the
>singleton. after that has happened, everything is back to normal and
>the singleton classes behave as expected and do not create any new
>objects.
>
>does anyone know how to get "safe" singleton classes to get rid of
>that problem?
>
>-- 
>Michael