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Object Oriented Programming in A Level William Marsh [Original version co-authored with Melissa Bustamante]

Session Aim and Outline Se Outline Aims • Using classes: the Face • Be able to motivate the use of classes and objects • Attributes and the constructor • Be able to explain OOP in • Reflection : Decomposition and relations Abstraction and design Decomposition • Practical break • ...progression in OOP • Reflection: How versus Why • Be aware of issues for teaching • Progression OOP • Misconceptions • Python versus java

A Face Class: Becoming a User of Objects There are many examples of classes and object that are familiar

Us Using the Face Class • File class is This is a variable. What is a familiar from turtle import * the type of its value? example from Face import Face • Are we f1 = Face(0, 0) aware we f1.draw() Draw methods: use which face is objects? f2 = Face(-200, 0) drawn? f2.setSize(80) f2.draw()

Obj Object ects and and Metho hods ds Name Type Description f1, f2 Variables; A drawing of a face Objects of ‘Face’ class Face Class name; constructor Create a Face object setSize Method of ‘Face’ class Set size of the face draw Method of ‘Face’ class Draw the face ‘Method’ is an OO word for function

Su Summary: Us Using Objects • Face representation is hidden • Method act (read or update) on objects f2.setSize(80) The object to be A second parameter: acted on the size to set The method: implements the action

Te Teaching Functional Decomposition • You have already learnt about functions - How they work - How to use them • Is it easy or hard to learn about functions? - What aspects are easier? - What aspects are harder?

Reflection on: Abstraction on and Decom ompos osition on Motivation: What are we trying to achieve with classes?

Li Liskov and and Gu Guttag 1986 1986 – De Decom ompos osition on • A very small program consisting of no more than a few hundred lines can be implemented as a single monolithic unit. • However, as the size of the program increases such a ... structure is no longer reasonable ... • Instead the program must be decomposed into ... modules that together provide the desired function. • … how to decompose large programming problems into small ones … what kinds of modules are useful … [how] modules can be combined to solve the original problem

Tw Two Different Aims for Learning OOP How Why • How to use classes • Decomposing a problem using classes - Create a new object - Use objects as variables (e.g. in a - Which classes to use? list) - What makes a good class? • How to create (declare) new • How to do good abstractions classes - Analysis of the problem - Add method and attributes - … and constructors • How classes can interact • How to create sub-classes - Software design (inheritance)

Su Summary • Emphasis continuity between OOP and previous programming - Use of objects and methods explained - Abstractions implemented using functions • Program decomposition; problem abstraction - Distinguish between learning syntax and - … practicing abstraction and program design • OOP is a new solution to the goal of decomposition using abstraction - Comparison with use of functions

Practical Wor ork Drawing Faces: Exercises 1 and 2

Declaring You our Own Classes Key concepts

The The Faces es Ex Exampl ple • Using Python turtle graphics • Good points - Visual and ?? Engaging (creative) - Class versus object distinction - Incremental • Limitations - Not typical of OO design - Complexity of drawing a distraction

Cl Class De Declaration on Constructor from turtle import class Face: Attributes def __init__(self, xpos, ypos): self.size = 50 self.coord = (xpos, ypos) self.noseSize = 'normal' Method def setSize(self, radius): self.size = radius def draw(self): ... self.drawOutline() ...

Class De Cl Declaration on def goHome(self): penup() from turtle import goto(self.coord) setheading(0) class Face: def drawOutline(self): def __init__(self, xpos, ypos): penup() self.size = 50 forward(self.size) self.coord = (xpos, ypos) left(90) self.noseSize = 'normal' pendown() circle(self.size) def setSize(self, radius): self.goHome() self.size = radius def draw(self): ... self.drawOutline() ...

Defining a Con De Constructor or class Face: Constructor • Has a special name def __init__(self, xpos, ypos): • May have self.size = 50 parameters self.coord = (xpos, ypos) self.noseSize = 'normal' Constructor name Constructor • Don’t forget ‘self’ Always ‘self’ parameter Initialise attributes

Attributes – Good At Good Practice • Attributes are not declared class Face: - In Python, nothing is! def __init__(self, xpos, ypos): self.size = 50 self.coord = (xpos, ypos) • Good practice to initialise all self.noseSize = 'normal' attributes in the constructor - Getters do not fail def setSize(self, radius): - Clear what the attributes are self.size = radius - Do not add more

Practical Wor ork Drawing Faces: Exercise 3 onwards

Teaching OOP in Python on

Pr Program Structure and Complexity If & loops x = • Program grows more complex while x > : y = in structure print • Simpler elements remain Functions - If & loop à part of function Function def - Method à part of class Function def Main program • Initialise vars • Call functions Classes & objects class Friend: def __init__() class Town: def m1(a, b): def __init__() Main program def m1(a, b): • Create obj • Call methods

OOP OOP Concept Details Basic • Calling a method of an object Con Concepts Prerequisite mechanics • Class as a template for data knowledge: • Class as a collection of methods functions & parameters Constructors • Definition and use Interaction • Object as a value (variable, list item, …) • Object as an attribute value (has-a Prerequisite relationship) knowledge: • Object passed as a parameter basic Abstraction • Class as a domain concept mechanisms and modelling • Methods (and constructor) have parameters Inheritance • Superclass and subclasses • Constructor called using super() • Method inherited or overridden

Misconception Possible Evidence Also lack of Attributes in the wrong scope • Omission of self (assignment or use) prerequisite Confusion between class and • No objects created knowledge: object • Only one instance functions & • Inheritance rather than instance parameters Confusion between class and • Many classes – all simple attribute Objects only contain data • No encapsulation • Only get and set methods Objects do not interact • All code in single class • Classes defined but not imported • Objects not used as attributes • Objects never passed as parameters Believing objects are copied • Unnecessary copying not shared • Unexpected sharing

Py Python Issues for Teaching OOP Usual OOP Python • The attributes are declared • Nothing is declared • A class has a fixed set of • Attributes appear when assigned attributes to • Attributes can be hidden: access • Hiding is not enforced only by methods Use Python to teach OOP • Avoid some Python tricks • Use only a subset • … explain later

Python Py ve versus Ja Java • No declarations • Declarations • Values are typed • Static typing of variables - Variable types are dynamic • Run time type checking • Compile time type checking • Syntax with indentation • Syntax with braces { } • Permissive philosophy • Rigid philosophy

Su Summary • Object-oriented programming - Builds on more basic programming - A approach to program decomposition (decomposition take practice) - Previous experience learning decomposition • Progression: concepts not syntax - Proficiency with functions essential - Class versus object - Classes have attributes and methods - Constructor - Relationships between classes; objects as values - Inheritance • Python – some disadvantages