On 05/29/2015 02:06 PM, sohcahto...@gmail.com wrote:
On Friday, May 29, 2015 at 10:18:29 AM UTC-7, Ethan Furman wrote: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.
Hence the need for a metaclass, as the point is to operate on the class, not the instance.
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?
Yes. In the case of Enum, however, it takes any user-defined __new__, which is needed for creating the original instances, and replaces it with a __new__ that only returns the already defined ones. -- ~Ethan~ -- https://mail.python.org/mailman/listinfo/python-list
