Lecture 5

Object-oriented Programming

Subclasses and inheritance

MCS 275 Spring 2023
David Dumas

Lecture 5: Subclasses and inheritance

Reminders and announcements:

• There will be no Homework 2
• Homework 3 to be posted Thurs, due Tue Jan 31
• Project 1 has new deadline: Fri Feb 10

Improved Point2 and Vector2

I added new features to our plane module between lectures. These are explored a bit in Worksheet 3. Included:

• Can multiply Vector2 by integer or float
• abs(Vector2) gives length
• and more...

Inheritance

Instead of starting a class definition from scratch we can indicate that it should inherit all the methods and attributes of some other class. Then we only need to specify the changes.

If new class B inherits from existing class A in this way, we say:

• B is a subclass of A (or child of A)
• A is a superclass of B (or parent of B)

Why subclass?

Some common reasons:

• To change behavior of an existing class
  (e.g. a dict that only allows certain kinds of keys)
• To avoid duplication by moving common code to a superclass with several subclasses
• To formalize relationships between classes

Subclassing should express an "is-a" relationship. Dog and Cat might be subclasses of Pet.

Python subclass syntax

Specify a class name to inherit from in the class definition:

    class ClassName(SuperClassName):
    """Docstring of the subclass"""
    # ... subclass contents go here ...

Now, all the methods of the superclass are immediately available as part of self.

Class hierarchies

Inheritance patterns are often shown in diagrams. Lines represent inheritance, with the superclass appearing above the subclass (usually).

Live coding

Let's build a class hierarchy for a simple robot simulation.

Every type of robot will be a subclass of Bot.

Bot has a position (a Point), boolean attribute active, and method update() to advance one time step.

Subclasses give the robot behavior (e.g. movement).

Planned Bot hierarchy

• PatrolBot walks back and forth.
• WanderBot walks about randomly.
• DestructBot sits in one place for a while and then deactivates.

Robot simulation template

We haven't built any of the Bot subclasses yet, but I have already created:

• A start on module bots containing one class Bot. It sits in one place. In bots.py in the sample code repository.
• A script botsimulation.py to run the simulation and show it with simple text-based graphics.

super()

If you define a method in a subclass, it replaces any method of the same name in the superclass.

That's usually very helpful. But what if the replacement wants to call the method it is replacing?

super().method_name(...) is the syntax for this; it calls method_name(...) of the superclass even if that method is redefined in the subclass.

Warning

You only need super() under specific circumstances:

• Superclass and subclass have a method with the same name, and
• In the subclass, you need to call the superclass version of that method for some reason.

This is rare. More common: Needing to call a method of the superclass that isn't redefined in the subclass.

That's easier: Just use self.method_name(...).

from

The from keyword can be used to import individual symbols from a module into the global scope.

import mymodule
# ...
mymodule.useful_function()  # module name needed

from mymodule import useful_function
# ...
useful_function()  # no module name needed

Please use from very sparingly!