classmethods, class variables and subclassing
Steve Holden
steve at holdenweb.com
Thu Oct 20 17:17:28 EDT 2005
Andrew Jaffe wrote:
> 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!)?
You are experiencing this problem because you are using hard-wired class
names. Try using (for example) self.__class__. That way, even if your
method is inheroted by a subclass it will use the class of the object it
finds itself a method of. No need to use classmethods.
regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC www.holdenweb.com
PyCon TX 2006 www.python.org/pycon/
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