Programming for Engineers in Python

Lecture 5: Object Oriented Programming

Autumn 2011-12

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Lecture 4 Highlights • Tuples, Dictionaries • Sorting Lists • Modular programming • Data analysis: text categorization • Collect data • Most frequent words • How it is really done 2

Today • Quick review on functions • Object Oriented Programming (partially based on chapters 15-18 in the book Think Python, available in the site) 3

Functions (Quick Overview) 4

How to Calculate 5! + 3! + 6!?

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Problems • Same code duplicated 3 times • More elements  more code duplicates • The program becomes longer and more complicated • May cause bugs, cut & paste errors • Hard to understand what was the writer’s intension • What would we like?

• Write the code once!

• Use it several times with different arguments 6

Solution - Functions 7

Functions • A group of declarations and statements that is assigned a name • Effectively, a named statement block • Usually has a value • A sub-program • Inside a fucntion we can call other functions • Which can themselves use other functions, and so on… • Function output: • Return value to the calling function • If no value is to be returned, ‘ None ’ is returned 8

What are They Good For?

• Generalize a repeated set of instructions • We don’t have to keep writing the same thing over and over • Solve bugs once… • They can break your problem down into smaller sub-tasks • Easier to solve complex problems • They make a program much easier to read and maintain • Abstraction – we don’t have to know how a function is implemented to use it 9

In Short • Why do we need functions?

• Code reusability • Modularity • Abstraction 10

Examples 11

Passing Arguments to Functions • When a function is called, arguments’

values

are attached to function’s formal parameters

by order

, and an assignment occurs

before

execution • Values are

copied

to formal parameters 12

Passing Arguments to Functions • A reference is passed

by reference

• Example: lists • This explains why we can change an array’s content within a function a 4 5 6 7 8 9 13

Example 14

Pyhton Memory Model •

Stack

: local variables and arguments, every function uses a certain part of the stack • Stack variables “disappear” when scope ends •

Heap

: global variables and objects, scope independent •

Garbage Collector

• Partial description 15

How to Change a Variable via Functions?

• So how can a method change an outer variable?

• By its return value • By accessing

heap

-based memory (e.g., lists) 16

A Car • How would you represent a car?

• Parts / features: 4 wheels, steering wheel, horn, color,… • Functionality: drive, turn left, honk, repaint,… • In Python???

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Lets Start Simpler • How would you represent a library?

• Container of books • Each book: • Title • Author • ISBN number • Number of pages • Publisher 18

Printing a Book 19

What Would We Want?

• To group the definition of several variables under a single name – a new type • Somewhat similar to functions… • Using the new type as if it is part of the language: • Define variables, perform operation 20

Object-Oriented Programming (OOP) • Represent

problem-domain

entities using a

computer language

• When building a software in a specific domain, describe the different components of the domain as types and variables • Thus we can take another step up in

abstraction

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Class as a Blueprint A

class

is a blueprint of

objects

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Class as a Blueprint A

class

is a blueprint of

objects

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Classes as Data Types • Classes define types that are a

composition

other types and have unique

functionality

• An

instance

of a class is named an

object

• Every instance may contain: • Constructors • Attributes (data members / fields) • Methods of 24

Car Example • Members: 4 wheels, steering wheel, horn, color,… • Every car instance has its own • Methods: drive, turn left, honk, repaint,… • Constructors: by color (only), by 4 wheels, engine,… 25

How to Represent a Point in 2D?

• Alternatives: • Two variables x,y • Elements in a list / tuple • A new data type • • Creating a new type is a (little) more complicated, but has its advantages (to be apparent soon)

class

– a user defined type • How to represent a point?

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Creating a new Point (instantiation) blank is an

instance

of class Point 27

But Where is the Point?

• x does not exist “in” blank (nor y) • We want to be able to access x,y via Point instance 28

Constructors • A special function, defined in the class’s code that “produces” instances • Invoked automatically when an object is instantiated • Given as input whatever is required to produce an instance • What would a Point’s constructor accept as inputs?

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Constructors – More Technically • Defined within the class’s scope • Always named __init__ • Short for initialization • 2 underscores init 2 underscores • The first parameter would be

self

• To be explained later on • Note that when invoking (calling) the constructor, self is not passed as an argument • It is common for the parameters of __init__ to have the same names as the attributes 30

Attributes p1 Point x  y  3.0

4.0

• The variable p1 refers to a Point object • p1.x means “Go to object p1 refers to and get the value of x” • There is no conflict between a variable x and the attribute x 31

Instances and Functions • Objects can be passed as arguments to functions • Objects are mutable 32

Copying Objects •

is

operator indicates that p1 and p2 are not the same object, the default behavior of the == operator is the same as the

is

operator 33

Instances as Return Values 34

A Circle • How would you represent a circle object?

• Attributes?

• Constructor?

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Shallow / Deep Copy 36

Shallow / Deep Copy (Cont.) 37

Object Oriented Programming • Programs are made of object definitions and function definitions, and most of the computation is expected in terms of operations on objects • Each object definition corresponds to some object or concept in the real world, and the functions that operate on that object correspond to the ways real-world objects interact 38

Methods •

Method

class is a function that is associated with a particular • Examples in strings, lists, dictionaries, tuples • Difference between methods and functions: • Methods are defined inside a class definition • The syntax for invoking a method • A method is called through an instance 39

Example: Distance Between Two Points (function) 40

Example: Distance Between Two Points (method) 41

Example: In Circle 42

Next week: defining a new type that behaves as part of the language • More OOP • Implementation of Rational Numbers 43