I think the important difference between None and booleans wrt singleton behaviour is that things are often compared with None using "is", so it's quite important that there only be one instance of NoneType around, and it makes sense not to give people the false impression that they can create another one.
Re-using predefined True and False instances is more of a performance optimisation, however -- they're not usually compared for identity, or even compared at all for that matter. So there's no particular reason to disallow calling bool(). In fact, there's an excellent reason to *allow* calling it: not only is bool a type, it's also often used as a function to coerce things to a boolean value: >>> bool(['hello']) True There's no equivalent reason to make NoneType callable, as coercing something to None makes very little sense. -- Greg -- http://mail.python.org/mailman/listinfo/python-list
