On Sep 28, 9:45 am, Steven D'Aprano <[EMAIL PROTECTED] cybersource.com.au> wrote: > On Sat, 27 Sep 2008 11:12:00 -0700, Lie wrote: > > This is probably unrelated to Python, as this is more about design > > pattern. I'm asking your comments about this design pattern that is > > similar in functionality to Singleton and Borg: to share states. > > > I'm thinking about this design pattern (I don't know if anyone has ever > > thought of this pattern before): > > > class OddClass(object): > > def __init__(self): > > global OddClass > > OddClass = self > > def __call__(): > > return self > > I don't think that pattern works as you give it. I suggest you read the > thread "What do you call a class not intended to be instantiated", > started by me on the 21st of September, which covers a similar pattern.
In fact, that thread inspired this thread. > I'm afraid it's a rather long thread, with a lot of people > misunderstanding what I was asking, but still worth reading. If you only > have time to read one post, I suggest you read my reply to Ben Finney, > posted yesterday. ... before I decided probably this pattern is probably isn't the answer to that thread. > My own feeling is that both your OddClass and my class without instances > are probably solutions looking for a problem. Well, actually, no, that's > too strong: I think the concept of "Class Singleton" is a perfectly valid > solution to certain problems, but it competes with more well-known > solutions like modules and Borg (in Python) and Singletons (the hammer > available in Java and C++). As for which is better, that's partly a > matter of personal taste and partly a matter of familiarity. > > > It do have a problem though, that you can't do isinstance(a, OddClass) > > But you can say "a is OddClass", which is more appropriate for a > Singleton. > > > The problem with Borg is that it is not inheritable (in certain > > sense[1]) and only work with old-style class (which is due to be > > completely removed on py3k)[2] > > No, there is a new-style Borg. Read the comments > here:http://code.activestate.com/recipes/66531/ > > The new-style Borg is hardly more complicated than old-style: 6 lines > versus 4. -- http://mail.python.org/mailman/listinfo/python-list
