Python Class Attributes and Subclassing

Question

I've googled and played a while but with no results trying to do something like this:

class  A(object):
    cl_att = 'I am an A class attribute'

class  B(A):
    cl_att += ' modified for B type'

class  C(A):
    cl_att += ' modified for C type'


instA = A()
print insAt.cl_att
>>> I am an A class attribute

instB = B()
print instB.cl_att
>>> I am an A class attribute modified for B type

instC = C()
print instC.cl_att
>>> I am an A class attribute modified for C type

print instA.cl_att
>>> I am an A class attribute

In short words I want to be able to "use and then override" a class attribute from my parent class.

Answer 1

Reference the parent class attribute and concatenate to it:

class  A(object):
    cl_att = 'I am an A class attribute'

class  B(A):
    cl_att = A.cl_att + ' modified for B type'

class  C(A):
    cl_att = A.cl_att + ' modified for C type'

Class bodies are executed much like functions, with the local names forming the class attributes. cl_att doesn't exist in the new 'function' to create the bodies for B and C, so you need to reference the attribute on the base class directly instead.

Demo:

>>> class  A(object):
...     cl_att = 'I am an A class attribute'
... 
>>> class  B(A):
...     cl_att = A.cl_att + ' modified for B type'
... 
>>> class  C(A):
...     cl_att = A.cl_att + ' modified for C type'
... 
>>> A.cl_att
'I am an A class attribute'
>>> B.cl_att
'I am an A class attribute modified for B type'
>>> C.cl_att
'I am an A class attribute modified for C type'