Emulating Final classes in Python

Steven D'Aprano steve+comp.lang.python at pearwood.info
Wed Jan 18 02:05:33 EST 2017


On Wednesday 18 January 2017 15:42, Ethan Furman wrote:

[...]
> Both those cases are good candidates for disallowing subclassing.

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?



-- 
Steven
"Ever since I learned about confirmation bias, I've been seeing 
it everywhere." - Jon Ronson



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