Is it dangeous when using custom metaclass?

[Gregory Ewing]

jfong at ms4.hinet.net wrote:
> class Structure(metaclass=StructureMeta): ...
> 
> class PolyHeader(Structure): ...
> 
> As my understanding, the metaclass's __init__ was called when a class was
> created. In the above example, both the Structure and PolyHeader called it.
> My question is: because the PolyHeader inherited Structure, is it reasonable
> for PolyHeader to call this __init__ again? Will it cause any possible
> trouble?

It's reasonable for both to call it, because they're distinct
instances of StructureMeta, each of which need to be initialised.

Whether it will cause a problem depends on what StructureMeta's
__init__ is supposed to do. Presumably you want a given structure
class to start allocating its offsets where its base class left
off, in which case you may need to do something like this:

class StructureMeta(type):
     def __init__(self, clsname, bases, clsdict):
         if bases:
            offset = bases[0].offset # assuming there isn't more than one base
         else:
            offset = 0
         ...

(BTW, why do you use setattr() to set the offset attribute
instead of just doing self.offset = offset?)

-- 
Greg