On Wed, 18 Jan 2017 13:10:41 +1100, Steven D'Aprano wrote: > On Tuesday 17 January 2017 20:37, Antoon Pardon wrote: > >> Op 17-01-17 om 08:05 schreef Steven D'Aprano: >>> I wish to emulate a "final" class using Python, similar to bool: >>> >>> py> class MyBool(bool): >>> ... pass ... >>> Traceback (most recent call last): >>> File "<stdin>", line 1, in <module> >>> TypeError: type 'bool' is not an acceptable base type > [...] >> I find those kind of classes annoying as hell and nobody has ever given >> me a good reason for them. What good was it to change Lock from a >> factory function to a class if you can't subclass the result anyway. > > I'm not sure which Lock class you're referring to. > > >> The result will probably be that users that would prefer to subclass >> your class will monkey-patch it. Something like: >> >> class MyLock: >> def __init__(self): >> self.lock = Lock() >> >> def __getattr__(self, attr): >> return getattr(self.lock, attr) > > That technique is called "delegation", or sometimes "composition". > > >> So I wonder what reasons do you have prefering that your users >> monkey-patch your class instead of subclassing it? > > Since my class provides no useful behaviour, I don't think anyone will > seriously have any reason to subclass it.
if it provides "No Usefull behaviour" why does it even exist what useless behaviour does it provide? :-) (no mater how pointless there is always someone how will find a use, I even know someone who successfully sold a box of paper shredding on ebay!) > > Python has at least three singleton instances which are used purely as > abstract symbols: they have no state, and very little behaviour besides > a nice repr. They are None, NotImplemented and Ellipsis. I'm effectively > trying to make my own abstract symbol. > > But it's mostly a learning exercise. -- Good programmers treat Microsoft products as damage and route around them. -- From a Slashdot.org post -- https://mail.python.org/mailman/listinfo/python-list
