15.9. Multiple Inheritance¶
Many programming languages limit classes to at most one parent class. Python supports multiple parent classes by offering a feature called multiple inheritance. For example, we can have an Airplane class with a flyInTheAir() method and a Ship class with a floatOnWater() method. We could then create a FlyingBoat class that inherits from both Airplane and Ship by listing both in the class statement, separated by commas. Open a new file editor window and save the following as flyingboat.py:
>>> class Airplane:
>>> def flyInTheAir(self):
>>> print('Flying...')
>>> class Ship:
>>> def floatOnWater(self):
>>> print('Floating...')
>>> class FlyingBoat(Airplane, Ship):
>>> pass
The FlyingBoat objects we create will inherit the flyInTheAir() and floatOnWater() methods, as you can see in the interactive shell:
>>> seaDuck = FlyingBoat()
>>> seaDuck.flyInTheAir()
Flying...
>>> seaDuck.floatOnWater()
Floating...
Multiple inheritance is straightforward as long as the parent classes’ method names are distinct and don’t overlap. These sorts of classes are called mixins. (This is just a general term for a kind of class; Python has no mixin keyword.) But what happens when we inherit from multiple compli- cated classes that do share method names?
For example, consider the MiniBoard and HintTTTBoard tic-tac-toe board classes from earlier in this chapter. What if we want a class that displays a miniature tic-tac-toe board and also provides hints? With multiple inheri- tance, we can reuse these existing classes. Add the following to the end of your tictactoe_oop.py file but before the if statement that calls the main() function:
- class HybridBoard(HintBoard, MiniBoard):
pass
This class has nothing in it. It reuses code by inheriting from HintBoard and MiniBoard . Next, change the code in the main() function so it creates a HybridBoard object:
gameBoard = HybridBoard() # Create a TTT board object.
Both parent classes, MiniBoard and HintBoard , have a method named getBoardStr() , so which one does HybridBoard inherit? When you run this program, the output displays a miniature tic-tac-toe board but also provides hints:
--snip-- X.. 123 .O. 456 X.. 789 X can win in one more move.
Python seems to have magically merged the MiniBoard class’s getBoardStr() method and the HintBoard class’s getBoardStr() method to do both! But this is because I’ve written them to work with each other. In fact, if you switch the order of the classes in the HybridBoard class’s class statement so it looks like this:
class HybridBoard(MiniBoard, HintBoard):
you lose the hints altogether:
--snip-- X.. 123 .O. 456 X.. 789
To understand why this happens, you need to understand Python’s method resolution order (MRO) and how the super() function actually works.