Gregory Ewing at 2018/10/16 UTC+8 PM 2:01:01 wrote > jf...@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 -- https://mail.python.org/mailman/listinfo/python-list
