Lecture 26

Object-oriented Programming 2

Operator overloading

MCS 260 Fall 2021
David Dumas

Reminders

Review

Key concepts from Lecture 25

• class — A type in Python that combines data (attributes) and behavior (methods).
• instance or object — A value whose type is a certain class (e.g. "hello" is an instance of str)
• attribute — A variable local to an object, accessed as objname.attrname.
• constructor — The method named __init__ that is called when a new object is created. Often sets a bunch of attributes using self.attrname = ...

Goals for today

Improve our Rectangle and Circle classes.

Introduce operator overloading.

Circles and rectangles

Desired methods

For both object types:

• Uniform scale about center
• Translation by a vector

__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.

Equality

How is A==B evaluated when A and B are objects?

By default, it checks whether the names refer to the same object in memory. This is often not what you want.

Overloading

Python allows us to specify our own behavior for operators like ==. This is called operator overloading.

If method A.__eq__ exists, then A==B evaluates to the return value of A.__eq__(B).

isinstance

The built-in function isinstance(obj,cls) returns a bool indicating whether obj is an instance of the class cls, e.g. isinstance(7,int)

Using it sparingly. Remember, Python recommends EAFP rather that LBYL in most cases.

EAFP = Easier to Ask Forgiveness than Permission

LBYL = Look Before You Leap

Many operators can be overloaded, including:

List of many more in the Python documentation.

Overloading built-in functions etc.