[Python-ideas] Please consider adding an overrideable flag to abstractmethod

[Neil Girdhar]

Alternatively, the need for an overriding implementation to call super 
could be marked by a different decorator.  This would allow linters to 
check that subclasses call super when they should, which is a very useful 
check in my opinion.  Such a decorator could be called "@overrideable", and 
could be exposed in abc.   When used in combination with @abstractmethod, 
it could have the above behavior. 

On Sunday, February 17, 2019 at 5:28:49 AM UTC-5, Neil Girdhar wrote:
>
> Marking a method M declared in C with abstractmethod indicates that M 
> needs to be *implemented* in any subclass D of C for D to be instantiated.
>
> We usually think of overriding a method N to mean replacing one 
> implementation in some class E with another in some subclass of E, F.  
> Often, the subclass implementation calls super to add behavior rather than 
> replace it.
>
> I think that this concept of *implementing* is different than *overriding*.
>
> However, abstract classes can have reasonable definition, and should 
> sometimes be overridden in the sense that subclasses should call super.  
> For example, when inheriting from AbstractContextManager, you need to 
> *override* the abstractmethod (!) __exit__, and if you want your class to 
> work polymorphically, you should call super.
>
> This is extremely weird.  Understandably, the pylint people are confused 
> by it (https://github.com/PyCQA/pylint/issues/1594) and raise bad 
> warnings.
>
> It also makes it impossible for me to raise warnings in my ipromise (
> https://github.com/NeilGirdhar/ipromise) project.  See, for example, 
> https://github.com/NeilGirdhar/ipromise/blob/master/ipromise/test/test_overrides.py 
> classes Y and W, which ought to raise, but that would raise on reasonable 
> code.
>
> My suggestion is to add a rarely used flag to abstractmethod:
>
> class AbstractContextManager:
>     @abstractmethod(overrideable=True)
>     def __exit__(self, exc_type, exc_value, traceback):
>         pass
>
> This would set a flag on the method like __abstractmethod_overrideable__, 
> which could be checked by ipromise's @overrides decorator, pylint's call 
> check, and anyone else that wants to know that a method should be 
> overridden.
>
> Best,
>
> Neil
>
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