Question on ABC classes

[Ethan Furman]

On 10/22/20 9:25 AM, Julio Di Egidio wrote:

> Now, I do read in the docs that that is as intended,
> but I am not understanding the rationale of it: why
> only if there are abstract methods defined in an ABC
> class is instantiation disallowed?  IOW, why isn't
> subclassing from ABC enough?

Let's say you subclass from ABC:

   class Abstract(ABC):
       pass

Then you subclass from that:

   class Concrete(Abstract):
       pass

Then subclass from that:

   class MoreConcrete(Concrete):
       pass

If you do a

   issubclass(<any of the above classes>, ABC)

you'll get

   True

The idea behind abstract classes is the prevention of creating non-functional instances, which means if any abstract 
methods, properties, etc., are present in an abstract class, then it's instances will not be fully functional; 
contrariwise, if there are no abstract anythings in the class, then it is functional and there's no reason not to allow 
it to be created.

Put another way:  if ABC is anywhere in a class' parentage, then it is "abstract" -- the only way to tell if 
instantiating it is okay is by the presence/absence of abstract pieces in the class.

--
~Ethan~