Question

Consider these two classes:

class A(object):
    name = "A"

class B(A):
    name = "Child of " + A.name

Simple. A.name will be “A” and B.name will be “Child of A”.

But it seems wrong to hard code A.name into B‘s definition of name. I naturally want to write something like:

class B(A):
    name = "Child of " + super(B).name

But that raises a NameError with B not yet being defined in the expression super(B). (Also, I’m not sure whether it should/would be super(B) or super(B, B) or something else, but that’s a moot point given the NameError.)

What’s the right way to do this, i.e. use super in the definition of a class attribute?

Answer 1

It can’t be done inside the class definition, for the reason you discovered: inside the class definition, you don’t have access to the class itself, since it’s not defined yet.

You can do it with a class decorator, though (if you’re using a version of Python with class decorators, which I think is 2.6 and up):

>>> class A(object):
...     name = "A"
>>> def makeAttr(cls):
...     cls.name = "Child of " + super(cls, cls).name
...     return cls
>>> @makeAttr
... class B(A):
...     name = "B"
>>> B.name
'Child of A'