MRO theory

Steve Holden steve at
Wed Apr 11 21:09:17 CEST 2007

Clarence wrote:
> I'm having problems creating classes because of "can't create a
> consistent mro" problems.
> I noticed, in a test program, that if the base class list that I
> pass to type.__new__ is sorted (using default keys, so presumably
> sorting by the id's of the class objects), that the problem goes away.
> Now in this test, the id's (memory addresses) of the class objects
> are presumably in the same order as the chronological order that
> the classes were created in (since the program is simple and doesn't
> delete any objects or make any temporaries). This may sound weird,
> but I don't actually care what order the base classes appear in as
> long as they're all there.
> This makes me wonder, since I can't say I understand the theory
> (I read the paper referenced in typeobject.c source, my brain
> didn't _quite_ melt)
> behind mro creation, if it is the case that there cannot be a
> consistency problem if each class in the base list was created
> after all classes that precede it in the base list.
> Can any genius weigh in on this question?
Without wishing to lay claim to genius, I'd suggest that you don;'t 
write programs relying on multiple inheritance until you have a thorough 
understanding of its principles.

The inability of the interpreter to create a consistent mro has nothing 
to do with the order in which the base classes were created or where 
they appear in memory. It's probably because you have introduced a 
circularity: you are trying to define something based on A, which is 
based on B, which is based on A or something similar.

Steve Holden       +44 150 684 7255  +1 800 494 3119
Holden Web LLC/Ltd
Skype: holdenweb
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