Core Python Programming (2nd Edition)

(, , , , , )
*MRO Problems Caused by Diamonds
The classic class method resolution never gave folks too many problems. It was simple to explain and
easy to understand. Most classes were single inheritance, and multiple inheritance was usually limited to
mixing two completely discrete classes together. This is where the term mix-in classes (or "mix-ins")
comes from.
Why the Classic Classes MRO Fails
The unification of types and classes in 2.2 brought about a new "problem," and that is related to all
(root) classes inheriting from object, the mother of all types. The diagram of a simple multiple
inheritance hierarchy now formed a diamond. Taking some inspiration from Guido van Rossum's essay,
let us say that you have classic classes B and C, as defined below where C overrides its constructor but
B does not, and D inherits from both B and C:
class B:
pass
class C:
def __init__(self):
print "the default constructor"
class D(B, C):
pass
When we instantiate D, we get:
>>> d = D()
the default constructor
Figure 13-3 illustrates the class hierarchy for B, C, and D, as well as the problem introduced when we
change the code to use new-style classes:
Figure 13-3. Inheritance problems are caused by the appearance of the base
class required by new-style classes, forming a diamond shape in the
inheritance hierarchy. An instance of D should not miss an upcall to C nor
should it upcall to A twice (since both B and C derive from A). Be sure to read
the "Cooperative Methods" section of Guido van Rossum's essay for further
clarification.