On Friday, May 29, 2015 at 10:18:29 AM UTC-7, Ethan Furman wrote: > On 05/29/2015 10:03 AM, sohcahto...@gmail.com wrote: > > On Friday, May 29, 2015 at 9:02:06 AM UTC-7, Mike Driscoll wrote: > > >> I've been asked on several occasions to write about intermediate or > >> advanced topics > >> in Python and I was wondering what the community considers to be > >> "intermediate" or > >> "advanced". I realize we're all growing in our abilities with the > >> language, so this > >> is going to be very subjective, but I am still curious what my fellow > >> Python > >> developers think about this topic. > > > > Metaclasses. > > > > I've read about them. I still don't understand them, why you would want > > them, and what you gain from them. > > Metaclasses change the way a class behaves. > > For example, the new (in 3.4) Enum class uses a metaclass. > > class SomeEnum(Enum): > first = 1 > second = 2 > third = 3 > > The metaclass changes normal class behavior to: > > - support iterating: list(SomeEnum) --> [SomeEnum.first, SomeEnum.second, > SomeEnum.third] > - support a length: len(SomeEnum) --> 3 > - not allow new instances to be created: --> SomeEnum(1) is SomeEnum(1) > # True > > -- > ~Ethan~
Regarding the first two, you can implement __iter__ and __len__ functions to create that functionality, though those functions would operate on an instance of the class, not the class itself. As for the third, can't you override the __new__ function to make attempts to create a new instance just return a previously created instance? -- https://mail.python.org/mailman/listinfo/python-list
