raising an exception when multiple inheritance involves same base

[Michael Hines]

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).
Just before the return hc, print hc.__mro__ yields for the second call:
(<class '__main__.hc'>, <type 'hoc.HocObject'>, <type 'object'>)
wheras in the __new__, print cls.__mro__ yields:
(<class '__main__.Foo'>, <class '__main__.hc'>, <class '__main__.hc'>,
<type 'hoc.HocObject'>, <type 'object'>)
Thanks,
Michael

[Arnaud Delobelle]

Michael Hines <mi***********@yale.eduwrites:

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