Simple metaclass code failing

Piet van Oostrum piet at
Mon Jun 1 05:38:05 EDT 2009

>>>>> LittleGrasshopper <seattlehanks at> (L) wrote:

>L> On May 31, 3:59 pm, Carl Banks <pavlovevide... at> wrote:
>>> On May 31, 3:52 pm, LittleGrasshopper <seattleha... at> wrote:
>>> > This is some simple code which I got from Guido's paper on the
>>> > unification of classes and types, which Arnaud suggested to improve my
>>> > knowledge of metaclasses:
>>> > class M1(type):
>>> >     pass
>>> > class M2(M1):
>>> >     pass
>>> > class M3(M2):
>>> >     pass
>>> > class C1:
>>> >     __metaclass__ = M1
>>> > class C2(C1):
>>> >     __metaclass__ = M2
>>> > class C3(C1, C2):
>>> >     __metaclass__ = M3
>>> > It is failing when processing C3:
>>> > Traceback (most recent call last):
>>> >   File "", line 18, in <module>
>>> >     class C3(C1, C2):
>>> > TypeError: Error when calling the metaclass bases
>>> >     Cannot create a consistent method resolution
>>> > order (MRO) for bases C2, C1


>L> I guess the resulting MROs do not satisfy monotonicity, I
>L> just have to find out how to derive the MRO. I had the notion that it
>L> was constructed by listing the current class, followed by the MROs of
>L> each base in order, and then retaining the rightmost instance of each
>L> class in the MRO list, but I think that might be an
>L> oversimplification, since in this case that would be:
>L> (C1, object)
>L> (C2, C1, object)
>L> (C3, C2, C1, object)
>L> And I don't see any issues. But I'll read the paper to figure out what
>L> the problem is, thanks.

Here is exactly the problem. Merging the two MRO's as you state would
give C3, C2, C1, object, i.e. C2 before C1.

But your class definition: 

class C3(C1, C2):
says that C1 should be before C2. Conflict!!
Change it to class C3(C2, C1):

You see it has nothing to do with the metaclasses. The following code
gives the same error:

class C1(object):
class C2(C1):
class C3(C1, C2):

Piet van Oostrum <piet at>
URL: [PGP 8DAE142BE17999C4]
Private email: piet at

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