python class properties

Question

I'm trying to find the best way to extend a class variable. Hopefully an example of the method I've come up with so far will make this clear.

class A(object):
    foo = ['thing', 'another thing']

class B(A):
    foo = A.foo + ['stuff', 'more stuff']

So I'm trying to make the subclass inherit and extend the parent's class variable. The method above works, but seems a bit kludgey. I'm open to any suggestion, including accomplishing something similar using a completely different approach.

Obviously I can continue to use this method if need be, but if there's a better way I'd like to find it.

Answer 1

Could use a metaclass:

class AutoExtendingFoo(type):

    def __new__(cls, name, bases, attrs):
        foo = []
        for base in bases:
           try:
               foo.extend(getattr(base, 'foo'))
           except AttributeError:
               pass
        try:
            foo.extend(attrs.pop('foo_additions'))
        except KeyError:
            pass
        attrs['foo'] = foo
        return type.__new__(cls, name, bases, attrs)

class A(object):
    __metaclass__ = AutoExtendingFoo
    foo_additions = ['thing1', 'thing2']
    # will have A.foo = ['thing1', 'thing2']

class B(A):
    foo_additions = ['thing3', 'thing4']
    # will have B.foo = ['thing1', 'thing2', 'thing3', 'thing4']

class C(A):
    pass
    # will have C.foo = ['thing1', 'thing2']

class D(B):
    pass
    # will have D.foo = ['thing1', 'thing2', 'thing3', 'thing4']

Answer 2

I definitively would go for instance-properties. (if I got it right, they are not bound to be static for your case?!)

>>> class A:
...     @property
...     def foo(self):
...         return ['thin', 'another thing']
...
>>> class B(A):
...     @property
...     def foo(self):
...         return super().foo + ['stuff', 'thing 3']
...
>>> B().foo
['thin', 'another thing', 'stuff', 'thing 3']