Question

I have a project with the following class structure:

class A(object):
    elems = [1, 2, 3]

class B(A):
    pass

class C(B):
    elems = [20, 100]

class D(B):
    elems = [4, 5]

Currently, Python’s default inheritance behaviour is to overwrite the elems attribute in each subsequent attribute declaration, e.g.:

A.elems == [1, 2, 3]
B.elems == [1, 2, 3]
C.elems == [20, 100]
D.elems == [4, 5]

I would like to be able to access an additive list of these values. In other words, I would like to be able to retrieve the following lists:

A.collated() == [1, 2, 3]
B.collated() == [1, 2, 3]
C.collated() == [1, 2, 3, 20, 100]
D.collated() == [1, 2, 3, 4, 5]

I don’t know how to do this. Any help would be greatly appreciated.

Thanks in advance,
– B

Answer 1

The easiest solution here is to just be explicit:

class A(object):
    elems = [1, 2, 3]
    @classmethod
    def collated(cls):
      return sorted(cls.elems)

class B(A):
    pass

class C(B):
    elems = B.elems + [20, 100]

class D(B):
    elems = B.elems + [4, 5]

print "A.collated() == ", A.collated()
print "B.collated() == ", B.collated()
print "C.collated() == ", C.collated()
print "D.collated() == ", D.collated()

There are tricky things you could do to make this more automatic, but I don’t think you want to. (If you think you want to, first go read up on how either descriptors or metaclasses work, then come back with specific questions.)