[FIXED] TypeErrors using metaclasses in conjunction with multiple inheritance

Issue

I have two questions converning metaclasses and multiple inheritance. The first is: Why do I get a TypeError for the class Derived but not for Derived2?

class Metaclass(type): pass

class Klass(object):
    __metaclass__  = Metaclass

#class Derived(object, Klass): pass # if I uncomment this, I get a TypeError

class OtherClass(object): pass

class Derived2(OtherClass, Klass): pass # I do not get a TypeError for this

The exact error message is:

TypeError: Error when calling the metaclass bases Cannot create a consistent method resolution order (MRO) for bases object, Klass

The second question is: Why does super not work in this case(if I use __init__ instead of __new__, super works again):

class Metaclass(type):
    def __new__(self, name, bases, dict_):
        return super(Metaclass, self).__new__(name, bases, dict_)

class Klass(object):
    __metaclass__  = Metaclass

There I get:

TypeError: Error when calling the metaclass bases type.__new__(X): X is not a type object (str)

I'm using Python 2.6.

Solution

The second question has already been well answered twice, though __new__ is actually a staticmethod, not a classmethod as erroneously claimed in a comment...:

>>> class sic(object):
...   def __new__(cls, *x): return object.__new__(cls, *x)
... 
>>> type(sic.__dict__['__new__'])
<type 'staticmethod'>

The first question (as somebody noted) has nothing to do with metaclasses: you simply can't multiply inherit from any two classes A and B in this order where B is a subclass of A. E.g.:

>>> class cis(sic): pass
... 
>>> class oops(sic, cis): 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 sic, cis

The MRO guarantees that leftmost bases are visited before rightmost ones - but it also guarantees that among ancestors if x is a subclass of y then x is visited before y. It's impossible to satisfy both of these guarantees in this case. There's a good reason for these guarantees of course: without them (e.g. in old style classes, which only guarantee the left-right order in method resolution, not the subclass constraint) all overrides in x would be ignored in favor of the definitions in y, and that can't make much sense. Think about it: what does it mean to inherit from object first, and from some other class second? That object's (essentially nonexistent;-) definition of its several special methods must take precedence over the other class's, causing the other class's overrides to be ignored?

Answered By - Alex Martelli