Referring to the class name from a class variable where inheritance is involved

[Ian Kelly]

On Tue, Dec 6, 2011 at 8:57 AM, Paul  Moore <p.f.moore at gmail.com> wrote:
> I want to set up an inheritance hierarchy. The base class will define
> a string value which should include the class name, but I don't want
> people who inherit from my class to have to remember to override the
> value.
>
> If I do this using an instance variable, it's reasonably easy:
>
>>>> class Base:
> ...     def __init__(self):
> ...         self.key = 'Key_for_' + self.__class__.__name__
> ...     def display(self):
> ...         print self.key
> ...
>>>> class Inherited(Base):
> ...     pass
> ...
>>>> b = Base()
>>>> i = Inherited()
>>>> b.display()
> Key_for_Base
>>>> i.display()
> Key_for_Inherited
>
> Rather than having the key for every instance, I'd like to use a class
> variable, but I can't see how I'd make that work (a class variable
> which is inherited but has a different value in derived classes). I
> could use a classmethod,but that feels like even more overkill than an
> instance attribute.
>
> Is there a way of doing this via class variables or something, or more
> relevantly, I guess, what would be the idiomatic way of doing
> something like this?

How about a class property?

class classproperty(object):
    def __init__(self, fget):
        self.__fget = fget
    def __get__(self, instance, owner):
        return self.__fget(owner)

class BaseClass(object):
    @classproperty
    def key(cls):
        return "Key_for_" + cls.__name__

class DerivedClass(BaseClass):
    pass

assert BaseClass.key == BaseClass().key == "Key_for_BaseClass"
assert DerivedClass.key == DerivedClass().key == "Key_for_DerivedClass"

If you like, you can also expand classproperty to allow setters and
deleters like property does.

Cheers,
Ian