Metaclasses and class variables

[Thomas Heller]

Jan-Ole Esleben <esleben at gmail.com> writes:

> Hi!
>
> I am new to this list, and maybe this is a stupid question, but I
> can't seem to find _any_ kind of answer anywhere.
>
> What I want to do is the following:
> I want to insert a class variable into a class upon definition and
> actually use it during definition.
>
> Manually, that is possible, e.g.:
>
> class A:
>   classvar = []
>   classvar.append(1)
>   classvar.append(2)
>
> I don't want to explicitly set the variable, though. My idea was to
> write the following:
>
> class Meta(type):
>   def __new__(cls, name, bases, d):
>     d['classvar'] = []
>     return type.__new__(cls, name, bases, d)
>
> class Test:
>   __metaclass__ = Meta
>
>   classvar.append(1)
>   classvar.append(2)
>
> However, Python complains that the variable isn't defined; it can be
> found in the class dictionary _after_ definition, though, and then it
> can also be used. But where's the conceptual difference (to the manual
> approach)?

classvar is defined AFTER the class has been created.  So, this should
work:

class Test:
    __metaclass__ = Meta

Test.classvar.append(1)

Thomas