Lecture 4

Object-oriented Programming

Operator overloading

MCS 275 Spring 2023
Emily Dumas

Lecture 4: Operator overloading

Reminders and announcements:

• Syllabus update coming soon to account for strike
• Project 1 due date will be postponed, details TBA

Object-Oriented Programming

Today we're starting our unit on object-oriented programming (OOP).

We assume knowledge of: Class definitions, creating instances, accessing attributes, calling methods.

Need to review these? See:

• MCS 260 Lecture 25
• MCS 260 Fall 2021 OOP sample code

We DO NOT assume knowledge of: Subclasses, inheritance, operator overloading.

Review of some key concepts

• class — A structure that allows data (attributes) and behavior (methods) to be bundled together, e.g. Point2.
• instance or object — A value belonging to a class, e.g. p = Point2(5,7)
• attribute — data stored in an object, e.g. p.x
• method — function that is part of an object (uses and/or modifies its attributes), e.g. p.radius()

Special methods / overloading

In Python, built-in operations are often silently translated into method calls.

e.g.   A+B turns into A.__add__(B)

These special method names begin and end with two underscores (__). They are used to customize the way your classes work with built-in language features.

Using these to add special behavior for operators like +,-,* is called operator overloading.

Operator examples

List of many more in the Python documentation.

More special methods

Live coding

Let's work on

• Point2 — point in the plane (a location in 2D)
• Vector2 — vector in the plane (e.g. the displacement between two points)

Difference of two Point2s is a Vector2.

Can multiply a Vector2 by a float or add it to a Point2.

Point2 plus Vector2 is a Point2.

Language features used

• isinstance(obj,classname) -- returns bool indicating whether obj is an instance of the named class (or subclass thereof)
• NotImplemented -- Special value that operators should return if the operation is not supported

__add__ & __radd__

In evaluating A+B, Python first tries

A.__add__(B)

B.__radd__(A)

There are reflected versions of all the binary operations (e.g. __rmul__).

Overloading danger

Overloading is best used when a function or operator has a clear, natural meaning for a class.

If used too much or in unintuitive ways, it makes programs harder to understand.

Singletons

When a class is designed so that it only ever has one instance, the class (or the only instance of it) is called a singleton.

We've seen two of these so far:

• None, the only instance of NoneType
• NotImplemented, the only instance of NotImplementedType