Michael Hines <[EMAIL PROTECTED]> writes:
> Hello,
> I have a class factory that supports single inheritance but it is
> an error if the base appears twice. e.g
> class Foo(hclass(h.Vector), hclass(h.List)):
> should raise an exception since h.Vector and h.List are really the same
> extension class, HocObject.
>
> So far I have only been able to do this by iterating over __mro__ during
> creation of an object as in:
>
> def hclass(c):
> class hc(hoc.HocObject):
> def __new__(cls, *args, **kwds):
> m = False
> for x in cls.__mro__:
> if (type(x) == type(hc)):
> if m == True:
> raise HocError, 'Multiple inheritance...'
> m = True
> kwds.update({'hocbase':cls.htype})
> return hoc.HocObject.__new__(cls, *args, **kwds)
> setattr(hc, 'htype', c)
> return hc
>
> Is there a way to do the test earlier (e.g during creation of the Foo
> class when hclass is called the second time
> instead of during creation of a Foo object).
Yes, by giving hoc.HocObject a custom metaclass:
class MetaHocObject(type):
def __new__(cls, name, bases, attrs):
if len(bases) > 1:
raise Exception("Only single inheritance allowed")
return type.__new__(cls, name, bases, attrs)
class HocObject(object):
__metaclass__ = MetaHocObject
That'll forbid descendants of MetaHocObject having several bases:
>> class A(HocObject): pass
...
>>> class B(HocObject): pass
...
>>> class C(A, B): pass
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 4, in __new__
Exception: Only single inheritance allowed
>>>
HTH
--
Arnaud
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