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BLaCK BOX: tOpOLOGICaL SOrtING aND pYthON’S MrO
the kind of structural ordering we’ve been working with in this section is actually an integral part of python 
object-oriented inheritance semantics. For single inheritance (each class is derived from a single superclass), 
picking the right attribute or method to use is easy. simply walk upward in the “chain of inheritance,” first 
checking the instance, then the class, then the superclass, and so forth. the first class that has what we’re 
looking for is used.
however, if you can have more than one superclass, things get a bit tricky. Consider the following example:
 
>>> class X: pass
>>> class Y: pass
>>> class A(X,Y): pass
>>> class B(Y,X): pass
 
If you were to derive a new class 
C
 from 
A
 and 
B
, you’d be in trouble. You wouldn’t know whether to look for 
methods in 
X
 or 
Y
.
In general, the inheritance relationship forms a daG (you can’t inherit in a cycle), and in order to figure out where 
to look for methods, most languages create a linearization of the classes, which is simply a topological sorting 
of the daG. recent versions of python use a method resolution order (or Mro) called C3 (see the references for 
more information), which in addition to linearizing the classes in a way that makes as much sense as possible 
also prohibits problematic cases such as the one in the earlier example.

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Chapter 4 
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 InduCtIon and reCursIon ... and reduCtIon 
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