Is it dangeous when using custom metaclass?

[jfong at ms4.hinet.net]

Gregory Ewing at 2018/10/16 UTC+8 PM 2:01:01 wrote
> 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.

The PolyHeader is already initialized by inheritance. Is there any way to bypass this __init__?

> 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?)

No particular reason, just follows the same code pattern ahead of it where the attribute names are from a list:-)

--Jach