Emulating Final classes in Python

Wed Jan 18 11:24:26 EST 2017

On 01/17/2017 11:05 PM, Steven D'Aprano wrote:
> I've given a metaclass that disallows subclassing:
>> class MyClass(MyParent, metaclass=FinalMeta):
> ...
>>> Ethan took that one step further by giving a class you inherit from to disallow
> subclassing:
>> class MyClass(MyParent, Final):
> ...
>>> Could we solve this problem with a decorator?
>>> @final
> class MyClass(MyParent):
> ...
>>> Without needing to add any special magic to MyParent or MyClass, apart from the
> decorator, can we make MyClass final? That would (in principle) allow us to
> make a subclass, and *then* set the class final so that no more subclasses
> could be made.
>>> I thought that the decorator could simply set the class' metaclass:
>> def final(cls):
> if cls.__class__ is type:
> cls.__class__ = Meta
> return cls
> raise TypeErrror('Possible metaclass conflict')
>>> but that's disallowed. Any ideas?

You still need to have the FinalMeta type and Final class available, but to use a decorator you'll need to scavenge the bits from the old class to make a new class of the same name and return that:
def final(cls):
 new_cls = Meta(cls.__name__, (Final, )+cls.__bases__, dict(cls.__dict__))
 return new_cls
Not sure if more is needed to handle __slots__, but this should get us started.
--
~Ethan~