ME 4990: Intro to CS�Object-Oriented Programming & Machine Learning 101

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Class!

Outline

• Class and attribute (AD Chp 15)
• Introduction to class and attributes
• Thoughts of designing a class (object-oriented modeling, object-oriented modeling)
• Python syntax for class definition and instantiation
• Methods in class

A type of thing

• Are there anything in common?
• If you are a DMV manager, how you are going to store the license?

https://www.deviantart.com/fairytalesartist/art/Partners-of-Zootopia-Driver-License-714373148

A type of thing

• If you are a DMV manager, how you are going to store the license? Use List?

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• Doesn’t show the common points of a DL
• Hard to operate

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https://www.deviantart.com/fairytalesartist/art/Partners-of-Zootopia-Driver-License-714373148

A type of thing

• We can consider Zootopia DL is a class
• Each animal’s DL is an instance of the class
• All instances have the same attributes
• Each attributes usually have the same type of value (e.g. integer, binary, etc.)*

* Not strict in Python, but it is a common practice

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A type of thing

• Zootopia DL is a class
• Zootopia DL class includes the following attributes:

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• Name
• Town
• Expiry
• DL Number
• Gender
• Species
• Fur
• Type
• Eye

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Define a class

• Define a class in Python

A type of thing

• Your turn: Get your California Driver’s License
• Name the attributes
• Modify the Zootopia_eg

https://www.sfchronicle.com/travel/article/California-Driver-License-A-new-look-and-12471176.php

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Outline

• Class and attribute (AD Chp 15)
• Introduction to class and attributes
• Thoughts of designing a class (object-oriented modeling, object-oriented modeling)
• Python syntax for class definition and instantiation
• Methods in class
• Introduction to methods
• Special methods __init__(), __str__(), and overload

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Modeling

• Referred as “object-oriented modeling” or “object-oriented design” (OOD)
• How are you going to model the following geometry using class, what attributes you should set?

Square

Rectangle

Circle

Modeling

• How are you going to model the following geometry using class, what attributes you should set?
• Should area be an attribute?

Square:

-length

Rectangle:

-length

-width

Circle:

-radius

Modeling

• We usually only include “atomic attribute” in a class (e.g. length for square)
• We usually do NOT include “derived attribute” in a class (e.g. area for square, which can be derived from length)

Square:

-length

Rectangle:

-length

-width

Circle:

-radius

Design Question

• If you are asked to design a class called “university” for Department of Education to manage funding of each university
• Which attribute you will put in?
• Which attribute are “derived” that you do not want to put in?

Outline

• Class and attribute (AD Chp 15)
• Introduction to class and attributes
• Thoughts of designing a class (object-oriented modeling, object-oriented modeling)
• Python syntax for class definition and instantiation
• Methods in class

Class “Point”

• Point class
• What attributes you want to set for a 2d point?

greenPoint

redPoint

Class “Point”

greenPoint

redPoint

Class declaration

Constructor, must have, do not omit self!

List all attributes with default value is a good habit

Class “Point”

greenPoint

redPoint

Instantiate an instance of a point

Assign values to the instance

Instantiate another instance of a point

Instantiate another instance; this instance uses default attribute values

Class inside a class

Rectangle class

Copying aliasing

Copying

Outline

• Class and attribute
• Method (AD Chp 17)
• Introduction and Syntax
• Special methods __init__(), __str__(), and overload

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Class

• Why we need class?
• Easier logic in programming
• Reuse the code
• Let others be able to use your blackbox

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Method: Introduction

• We do not want to set area as an attribute
• But how can we calculate the area?
• In the main program?

Method

• As a class author, we want to provide a good method for everyone to use!
• We not only provide attribute, but also provide methods (functions)
• Just like your parents not only give you money, but also teach you how to use money

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Image from online source

Method

• We define get_area() method

Method Syntax 101: method without argument

• When using a method in main() program

Declare a method

Using def

self must be there!

To access the attribute and methods of this class, use self.

objectName.method(), no self needed

Method Syntax 102: method with argument

• When using a method in main program

Declare a method

Using def

self must be there as the first argument

To access the attribute and methods of this class, use self.

newX and newY are arguments of the method (with default), no self on argument of the method

objectName.method()

No self needed. Put arguments here, best practice to list argument names

Your turn

• Add a method to calculate the perimeter of the rectangle, return perimeter
• Add a method to update the width, height of the rectangle, return updated area and perimeter

Method in method

• We can re-use a method in a class for another method

Use self.methodName() to call a method within the same class.

No self in the argument. If the method has other arguments, you should pass them here

Outline

• Class and attribute
• Method (AD Chp 17)
• Introduction and Syntax
• Special methods __init__(), __str__(), and overload

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Constructor __init__(self)

greenPoint

Constructor __init__(self, args)

• Constructor is commonly used to let user’s to specify values of each attribute at start:

__str__(self):

• If you want to use print() command
• How to print “pointName?”

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Operator overloading

• You just want to use “+”

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Practice

• Design a Zootopia license class
• The class has a method, called “isFastAnimal”
• If species is “sloth” and name is “flash”, return True
• Otherwise return False

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Practice

• Design a Zootopia license class
• The class has a method, called “isFastAnimal”
• If species is “sloth” and name is “flash”, return True
• Otherwise return False

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Practice 2

Do exercise at:

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https://greenteapress.com/thinkpython2/html/thinkpython2016.html#sec186

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