class B(object):pass
class D1(B):pass
class D2(D1):pass
class D(D1, D2):pass Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: Error when calling the metaclass bases
Cannot create a consistent method resolution
order (MRO) for bases D1, D2
Is this as intended? Especially since reversing the order makes it OK:
class D(D2, D1):pass
D.__mro__
(<class '__main__.D'>, <class '__main__.D2'>, <class '__main__.D1'>,
<class '__m
ain__.B'>, <type 'object'>)
Why should order of base classes matter? This only affects types,
old-style classes are unaffected.
The workaround to this problem (if it is a problem and not a feature
that I'm misunderstandin g) is to put more-derived classes at the front
of the list. I ran into this with dynamically-created classes, and
sorting the bases list with the following comparator put them right:
def cmpMRO(x,y):
if x == y:
return 0
elif issubclass(x, y):
return -1
elif issubclass(y, x):
return +1
else:
return cmp(id(x), id(y))
Incidentally, is it safe to have an mro() method in a class, or is this
as reserved as the usual __reserved_word s__? Does Python use mro() (as
opposed to __mro__) internally or anything?
Thanks
John
