You may sometime need to create classes that can only be instantiated
once. There are many ways to create these so-called "Singleton"
classes. Refer to this site_ and also_ for example code. One way not
mentioned in these links (code is from here_) is::

    class MySingleton_:
        def __init__(self):
            # Do something
            pass
        def __call__(self):
            return self

MySingleton_ = MySingleton_()
</verbatim>

.. _this site: http://www.norvig.com/python-iaq.html
.. _also: http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/52558
.. _here: http://www.faqts.com/knowledge_base/view.phtml/aid/4282

-- EdmundLian_ - 21 Jan 2002 

Note that all these singleton ideas are equivalent to a module-level
global -- the only advantage is that they allow using a class-like
syntax.  Like a module-level global, if you have ambiguities in your
path such that a module is imported twice, the object will not be a
true singleton.  Also, like a global, you must be particularly careful
about thread-safety.

Singleton's are in some ways a case of using a factory: that is,
instead of normal class instantiation, you use something that looks
like instantiation but may return an already-created object.  I've
written a small class that assists in creating factories:
ParamFactory.py_ -- useful for the case when you have an external ID
(e.g., a username), and you want to be sure you create only one object
for each ID.

.. _ParamFactory.py: http://www.colorstudy.net/software/python/ParamFactory_.py

-- IanBicking_ - 21 Jan 2002