Lecture 23

Reminders

Custom types in Python

In Python, classes are the way to define your own types. A value of that type is an object or instance.

Analogy: class "Cat", instance "Mr. Mittens".

Objects bundle together data and behavior (things you can do with a specific sort of data).

Sample Problem

Suppose we are writing programs that will work with geometric objects in the plane, such as circles and rectangles.

How should we represent these objects as numeric data?

Representation

But what type should we use? list, tuple, dict?

Classes

We can create our own type called Circle, using a class definition.

By convention class names LookLikeThis (capitalized words with no separator).

Classes are mutable and can contain named attributes, which are like variables.

Circle() will create a new object of type Circle.

Acting on objects

Now imagine that moving geometric objects right and left (x-translation) is important in our program. How would we do it?

We could create functions that modify the objects:

circle_translate_x(circle,delta_x)
rectangle_translate_x(rectangle,delta_x)            

Methods

Notice the functions we just defined take an object as the first argument and modify it in some way?

This is so common that there is a language feature just for this purpose.

A method is a function that is defined inside a class, and which then exists in each object of that type using syntax like C.translate_x(dx) if C is an object of type Circle.

Important note

A method is called like this: C.translate_x(dx)

But it receives argument list: (C, dx)

Methods always receive the object as their first argument!

__init__

For a class Circle, when we call Circle() we are actually running a special method called the constructor. It sets up a new object for us.

If we define a method __init__(self,...) in a class, it becomes the constructor.

__str__

When Python needs to convert an object to a string, it calls the __str__(self) method, if it exists.

Define this and return a string that is a human-readable representation of what the object is.