MRO Error on Multiple Inheritance?

Ben Finney bignose+hates-spam at benfinney.id.au
Sat Jan 5 00:13:07 CET 2008


Ming <minger at gmail.com> writes:

> I'm working through Wesley Chun's CPP2e and got this error on 13.11.1,
> pp 548 where his interpreter snippet shows no problems:

I don't know what a "CPP2e" is. Is it a book? Can you give the ISBN?

> ActivePython 2.5.1.1 (ActiveState Software Inc.) b
> Python 2.5.1 (r251:54863, May  1 2007, 17:47:05) [
> win32
> Type "help", "copyright", "credits" or "license" f
> >>> class A(object): pass
> ...
> >>> class B(A): pass
> ...
> >>> class C(B): pass
> ...
> >>> class D(A, B): pass
> ...
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: Error when calling the metaclass bases
>     Cannot create a consistent method resolution
> order (MRO) for bases A, B
> 
> (I submitted the problem to the author but I'm not sure I'll ever hear
> back.)  I'm guessing that this kind of diamond inheritance is
> prohibited by the interpreter, and that his lack of error messages
> from the interpretation is due to actually leaving out the "class
> B(A): pass"  Can someone shed light?  Thanks.

That's not an example of diamond inheritance
<URL:http://en.wikipedia.org/wiki/Diamond_problem> because classes A
and B are not distinct classes with a *common* base. Instead, they're
in a direct parent-child relationship.

So there's no sense in defining class D to inherit from both A *and*
B. To get a descendent of both those classes, inheriting from B is
sufficient. It should rather be::

    class D(B): pass

-- 
 \     "Pinky, are you pondering what I'm pondering?" "Uh, I think so, |
  `\     Brain, but we'll never get a monkey to use dental floss."  -- |
_o__)                                            _Pinky and The Brain_ |
Ben Finney



More information about the Python-list mailing list