Emulating Final classes in Python

Ethan Furman ethan at stoneleaf.us
Wed Jan 18 17:02:47 EST 2017


On 01/18/2017 08:24 AM, Ethan Furman wrote:
> 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.

One problem with the above is existing instances won't be modified to inherit from the updated class.  I am unsure if that is solvable before 3.6, but in 3.6 one can use the new __init_subclass__ to avoid a Final base class, a FinalMeta type, and just update the existing class:

def final(cls):
     def init_subclass(cls, **kwargs):
         raise Exception('Final class cannot be subclassed')
     cls.__init_subclass__ = classmethod(init_subclass)
     return cls

This can be used as a decorator at class creation time, or at any later date to lock down a class.  The downside is it's less obvious that the class is final... meaning there are no clues in the MRO.

--
~Ethan~


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