(python2.1-tut.info)Multiple Inheritance


Multiple Inheritance
--------------------

Python supports a limited form of multiple inheritance as well.  A
class definition with multiple base classes looks as follows:

     class DerivedClassName(Base1, Base2, Base3):
         <statement-1>
         .
         .
         .
         <statement-N>

The only rule necessary to explain the semantics is the resolution rule
used for class attribute references.  This is depth-first,
left-to-right.  Thus, if an attribute is not found in
`DerivedClassName', it is searched in `Base1', then (recursively) in
the base classes of `Base1', and only if it is not found there, it is
searched in `Base2', and so on.

(To some people breadth first -- searching `Base2' and `Base3' before
the base classes of `Base1' -- looks more natural.  However, this would
require you to know whether a particular attribute of `Base1' is
actually defined in `Base1' or in one of its base classes before you
can figure out the consequences of a name conflict with an attribute of
`Base2'.  The depth-first rule makes no differences between direct and
inherited attributes of `Base1'.)

It is clear that indiscriminate use of multiple inheritance is a
maintenance nightmare, given the reliance in Python on conventions to
avoid accidental name conflicts.  A well-known problem with multiple
inheritance is a class derived from two classes that happen to have a
common base class.  While it is easy enough to figure out what happens
in this case (the instance will have a single copy of "instance
variables" or data attributes used by the common base class), it is not
clear that these semantics are in any way useful.


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