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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