>>> class A(object):
...   m = 1
...
>>> class B(A): pass
...
>>> class C(A): pass
...
>>> class D(A,B): pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: MRO conflict among bases A, B

I happen to know what MRO stands for, but probably 
most people that made the typo I did in the diamond 
inheritance graph will have no idea what they did wrong...

How about "One of the declared superclasses A, B 
inherits from the other, the method resolution order 
(MRO) would therefore be undefined. Cannot create 
class."

-- bjorn

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I don't think the first part of the sentence is accurate.
MRO conflicts can arise for less straight-forward reasons.
How about:

"Cannot create class.  Superclasses A, B have conflicting 
inheritance trees which leave the method resolution order 
(MRO) undefined."

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Even better. If I were to suggest anything, it would be to 
add "(lookup)" after "resolution" since that seems to be the 
more frequently used terminology -- not a big issue though.

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Fixed.  See:

Objects/typeobject.c 2.216
Lib/test/test_descr.py 1.187

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Unfortunately the new error message makes no sense when you
get it by rearranging __bases__.

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Idea 1
------
TypeError: Cannot create class.  
Accessing superclasses A, B in order creates conflicting
inheritance trees which leave the method resolution order 
(MRO) undefined.

Idea 2
------
Cannot create a consistent method resolution order (MRO) 
for bases A, B.

Idea 3
------
Doh!

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Michael, do you have a preference?

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I'd say "idea 2".  the issues too complicated to have a hope
of explaining in the error message, so the only hope is to
hand out enough key words to make finding the relavent
section of the docs easy.

I'd also like to say:

The "of course, while I have no problem with this at all,
it's surely too much for a lesser being" flavor of argument
always rings hollow to me. -- Tim Peters, 29 Apr 1998

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Fixed.