Classes

COMPSCI 105 SS 2015Principles of Computer Science

COMPSCI 1052

Exercise Work on PreLab01 What is the output of the following code

fragment?

Lecture03

x = ['a', 'b', 'c']y = x z = ['a', 'b', 'c']print (x is y)print (x == y)print (x is z)print (x == z)

COMPSCI 1053

Classes Python has a number of classes built-in

lists, dictionaries, sets, int, float, boolean, strings

We can define our own classes creates a new type of object in Python

Classes consist of: state variables (sometimes called instance variables) methods (functions that are linked to a particular

instance of the class)

class name_of_the_class: definition of the class goes here

Lecture03

COMPSCI 1054

Example An example:

Instantiating Classes A class is instantiated by calling the class object:

class foo: a, b, c = 0, "bar", (1,2)

i = foo()print (i.a)print (i.b)print (i.c)

0bar(1, 2)

Lecture03

Example01.py

COMPSCI 1055

The simplest class possible Note: “Pass” is a statement that does nothing

It is often used as a placeholder when developing codeclass Point: pass

>>> p = Point()>>> p<__main__.Point object at 0x02702570>>>> p.x = 2>>> p.y = 4>>> p.x2>>> p.y4

Lecture03

Example02.py

COMPSCI 1056

Saving the class Classes are designed to help build modular code

Can be defined within a module that also contains application code

Multiple classes can be defined in the same file In this course, we will typically store each class in

their own module To use the class in another module, you will need to

import the moduleclass Point: ...

Saved in a file called Geometry.py

from Geometry import Point

p = Point(5,7)

Use the class like this

px: 5

y: 7

The object

in memor

y

Lecture03

Geometry.py

COMPSCI 1057

Setting the initial state of the object We want to define the Point class so we can write

code that sets the initial values of some variables

First, we need to define a special method of the Point class called a constructor The constructor is called whenever you create an

object of the Point class.

from Geometry import Point

p = Point(5, 7)

Lecture03

COMPSCI 1058

Constructors Each class should contain a constructor method

Name of the method is __init__ The method always has at least one parameter, called

self Self is a reference to the object that we are creating The constructor can have other parametersclass Point:

def __init__(self, loc_x, loc_y): self.x = loc_x self.y = loc_y

from Geometry import Point

p = Point(5, 7)print(p.x)print(p.y)

Lecture03

Saved in a file called Geometry.py

COMPSCI 1059

Adding functionality Defining more methods

A method to shift a point by a given amount in horizontal and vertical directions

Note: the method is named normally, but has the additional parameter (self) as the first parameter All methods that are called on an instance of an object need the

self parameter

class Point: def __init__(self, loc_x, loc_y): self.x = loc_x self.y = loc_y

def translate(self, dx, dy): self.x += dx self.y += dy

Lecture03

COMPSCI 10510

Using the Point class Methods are defined to accept self as the first

parameter

We call the method using: object_name.method(params)

class Point: def __init__(self, loc_x, loc_y): self.x = loc_x self.y = loc_y

def translate(self, dx, dy): self.x += dx self.y += dy

from Geometry import Pointp = Point(0,0)p.translate(10,5)

Lecture03

COMPSCI 10511

Exercise 1 Define a class that will be used to represent a

square with a given side length. Your class should include a constructor that will allow

the square to be used as follows:

Add a method to the class to calculate the perimeter of the square. The following code shows how the method may be used.

from Geometry import Squareside = 10s = Square(side)

print (s.perimeter())

Lecture03

40

COMPSCI 10512

Write a class to represent fractions in Python create a fraction add subtract multiply divide text representation

Example: Fractions

½numeratordenominator

Lecture03

COMPSCI 10513

Model of objects in memory

methods

state

num:

den:

7

8

methods

state

num:

den:

3

4

methods

state

num:

den:

1

2

xy

z

from Fraction import Fraction

x = Fraction(1,2)y = Fraction(3,4)z = Fraction(7,8)

Lecture03

Example03.py

COMPSCI 10514

All classes must have a constructor The constructor for a Fraction should store the

numerator and the denominator

Constructor

class Fraction: def __init__(self, top, bottom): self.num = top #numerator self.den = bottom #denominator

Lecture03

COMPSCI 10515

So far, we can create a Fraction

We can access the state variables directly Although not generally good practice to do so

What else can we do with Fractions? Nothing yet. We need to write the functions first!

Using the Fraction class

>>> x.num3>>> x.den4

>>> x = Fraction(3, 4)

Lecture03

Lecture 04

COMPSCI 10516

Overriding default behaviour All classes get a number of methods provided by

default Since default behaviour is not very useful, we should

write our own versions of those methods __repr__ __str__

Lecture03

COMPSCI 10517

Often we want to use a string that combines literal text and information from variables Example:

We can use string formatting to perform this task Use curly braces within the string to signify a variable

to be replaced

We can put the argument position in the curly braces

Aside: Use of string formatting syntax

name = 'Andrew'greeting = 'Hello ' + name + '. How are you?'

my_name = 'Andrew'greeting = 'Hello {name}. How are you?'.format(name=my_name)

first = 'Andrew'second = 'Luxton-Reilly'greeting = 'Hello {0} {1}'.format(first, second)

Lecture03

COMPSCI 10518

The __repr__ method produces an string that unambiguously describes the object All classes should have a __repr__ function

implemented Ideally, the representation could be used to create the

object For example, a fraction created using Fraction(2, 3) should

have a __repr__ method that returned 'Fraction(2, 3)'

Using the object

__repr__

class Fraction:def __init__(self, top, bottom): self.num = top

self.den = bottom def __repr__(self): return 'Fraction({0}, {1})'.format(self.num, self.den)

>>> x = Fraction(2, 3)>>> x

Fraction(2, 3)

Lecture03

COMPSCI 10519

Without the __repr__ method

>>> x = Fraction(2, 3)>>> x

class Fraction: def __init__(self, top, bottom): self.num = top self.den = bottom

<__main__.Fraction object at 0x02762290>)

Lecture03

COMPSCI 10520

The __str__ method returns a string representing the object By default, it calls the __repr__ method The __str__ method should focus on being human

readable We should implement a version with a natural

representation:

After we have implemented the method, we can use standard Python

__str__

def __str__(self): return str(self.num) + '/' + str(self.den)

>>> x = Fraction(3, 4)>>> print(x)3/4

Lecture03

COMPSCI 10521

Without the __str__ method

>>> x = Fraction(2, 3)>>> print(x)

class Fraction: def __init__(self, top, bottom): self.num = top self.den = bottom

<__main__.Fraction object at 0x02714290>

Lecture03

COMPSCI 10522

Exercise 2 Write the __repr__ method for the Square class

created earlier.

Would it be useful to implement a __str__ method?

What would you choose to produce as output from a __str__ method?

Lecture03