classmethods, class variables and subclassing

[Andrew Jaffe]

Hi,

I have a class with various class-level variables which are used to 
store global state information for all instances of a class. These are 
set by a classmethod as in the following (in reality the setcvar method 
is more complicated than this!):

class sup(object):
     cvar1 = None
     cvar2 = None

     @classmethod
     def setcvar1(cls, val):
         cls.cvar1 = val

     @classmethod
     def setcvar2(cls, val):
         cls.cvar2 = val

     @classmethod
     def printcvars(cls):
	print cls.cvar1, cls.cvar2


I can then call setcvar on either instances of the class or the class 
itself.

Now, the problem comes when I want to subclass this class. If I 
override the setcvar1 method to do some new things special to this 
class, and then call the sup.setcvar1() method, it all works fine:

class sub(sup):
     cvar1a = None

     @classmethod
     def setcvar1(cls, val, vala):
         cls.cvar1a = vala
         sup.setcvar1(val)

     @classmethod
     def printcvars(cls):
         print cls.cvar1a
         sup.printcvars()

This works fine, and sets cvar and cvar2 for both classes.

However, if  I *don't* override the setcvar2 method, but I call 
sub.setcvar2(val) directly, then only sub.cvar2 gets set; it is no 
longer identical to sup.cvar1!

In particular,
     sub.setcvar1(1,10)
     sub.setcvar2(2)
     sub.printcvars()
prints
   10
   1 None

i.e. sub.cvar1, sub.cvar1a, sub.cvar2= 1 10 2
but sup.cvar1, cvar2= 1 None

since sup.cvar2 has never been set, and this is what sup.printcvars() 
looks for.

This behavior is "expected", but is it desirable?

For my application, at least, I think the problem really comes in the 
printcvars method: is there any way to call the overridden 
sup.printcvars() but with, effectively, cls=sub?

Thanks for reading this far!

Andrew


Andrew




This seems like a bug

Is this expected behavior, or a bug (or both -- it is expected but 
probably not what is wanted!)?