mixins and new style classes

[mk]

 >>> class Person(object):
...     pass
...
 >>> class Friendly(object):
...     def hello(self):
...             print 'hello'
...
 >>>
 >>> Person.__bases__ += (Friendly,)
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: Cannot create a consistent method resolution
order (MRO) for bases object, Friendly




But it works for old-style classes:


 >>> class Person:
...     pass
...
 >>> class Friendly:
...     def hello(self):
...             print 'hello'
...
 >>>
 >>> p = Person()
 >>>
 >>> Person.__bases__ += (Friendly,)
 >>>
 >>> p.hello()
hello


Python is 2.6.1.