simple metaclass question

[Michael Hudson]

Blair Hall <b.hall at irl.cri.nz> writes:

> Please don't ask me why, but in trying to use metaclasses
> I simplified my code to the following:
> 
> class M(object):
>     def __init__(cls,name, bases,dict):
>         cls.val=0
> 
>     def set(cls,n):
>         cls.val = n

/Usually/ metaclasses want to inherit from type... not sure what
you're trying to achieve here, so I might be wrong.

> class A(object):
>     __metaclass__ = M
>     val = 3
> 
> class B(object) :
>     val = 2
> 
> I was then surprised to see that
> >>> A.val
> 0
> >>> B.val
> 2
> >>>A.set(5)
> >>>A.val
> 5
> 
> Why is the initial A.val not 3?

Because you smashed it in M.__init__?

> How should I change the code so that it
> does work that way?

Check the ns argument for a key 'val'?

> I am trying to eventually create
> classes that hold values and can be manipulated
> by arithmetic operators.
> 
> Any pointers gratefully accepted.

HTH,
M.

-- 
  This is the fixed point problem again; since all some implementors
  do is implement the compiler and libraries for compiler writing, the
  language becomes good at writing compilers and not much else!
                                 -- Brian Rogoff, comp.lang.functional