Intro To Object Oriented Programming

Introduction

Previously, we’ve talked about whether or not a function mutates (aka changes) a variable.

For example, we might want a function that takes a list as an input and mutates that list, OR we might want a function that takes a list as an input does NOT mutate that list, but instead tells us something about that list, or returns a new list with changes made to it.

This ideas also applies to methods! As we’ve talked about previously, methods often mutate (or change) the object they belong to. Think about our Pizza class from the lesson. The add_toppings method mutates a Pizza object by changing the value of self.toppings. On the other hand, the price method did not change the value of any attributes, so it does NOT mutate the Pizza object.

For this CQ, we are going to create and compare two similar methods. They both have similar functionality, but one mutates the class object and one does not.

0. Set up files

Create a new folder inside “lessons” called “CQ08” and a new file inside of it titled “point.py”.

1. Create Point class

We are creating a Point class that has both an x and a y attribute. Think of it as the representation of a point on an (x,y) coordinate graph. We are going to talk about methods that scale a point, or change its value by multiplying the x and y values by some value.

Create a class with the following properties:

• The class name is Point
• It has the attributes x and y which are both floats

Now, define a constructor (aka an __init__ method) that takes as input x_init: float and y_init: float and assigns those as the initial values for the attributes x and y.

2. Mutating Method: Point#scale_by

Now, you are going to write a method that belongs to the Point class and mutates a Point.

It’ll should the following properties:

• Method name: scale_by
• Parameters: self and factor: int (In general, the first parameter of a method should always be self)
• Return Type: None
• Behavior: It should update the x and y attributes so that x = x * factor and y = y * factor

3. Non-Mutating Method: Point#scale

Now, you are going to write a method that belongs to the Point class and instead of mutating a Point, it creates a new Point.

It’ll should the following properties:

• Method name: scale
• Parameters: self and factor: int (In general, the first parameter of a method should always be self)
• Return Type: Point *
• Behavior: It should return a new Point with x and y attributes equal to self.x * factor and self.y * factor.

*Reminder: if we are defining a method that returns its own type (e.g. a method in our Point class that returns a Point), we also need to add the line from __future__ import annotations to the top of our file. For an example, look back at our Pizza class!

Checking Your Methods For Correctness

You can check your methods by creating a new file in your CQ08 file called make_points.py and importing your Point class using from lessons.CQ08.point import Point. Then create a new point and call the methods.

4. Submission

Create your submission with the following command:

python -m tools.submission lessons/CQ08