Classes COMPSCI 105 SS 2015 Principles of Computer Science

Exercise  Work on PreLab01  What is the output of the following code fragment? Lecture03COMPSCI 1052 x = ['a', 'b', 'c'] y = x z = ['a', 'b', 'c'] print (x is y) print (x == y) print (x is z) print (x == z) x = ['a', 'b', 'c'] y = x z = ['a', 'b', 'c'] print (x is y) print (x == y) print (x is z) print (x == z)

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 class name_of_the_class: definition of the class goes here Lecture03COMPSCI 1053

Example  An example:  Instantiating Classes  A class is instantiated by calling the class object: class foo: a, b, c = 0, "bar", (1,2) class foo: a, b, c = 0, "bar", (1,2) i = foo() print (i.a) print (i.b) print (i.c) i = foo() print (i.a) print (i.b) print (i.c) 0 bar (1, 2) 0 bar (1, 2) Lecture03COMPSCI 1054 Example01.py

The simplest class possible  Note: “Pass” is a statement that does nothing  It is often used as a placeholder when developing code class Point: pass class Point: pass >>> p = Point() >>> p >>> p.x = 2 >>> p.y = 4 >>> p.x 2 >>> p.y 4 >>> p = Point() >>> p >>> p.x = 2 >>> p.y = 4 >>> p.x 2 >>> p.y 4 Lecture03COMPSCI 1055 Example02.py

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 module class Point:... class Point:... Saved in a file called Geometry.py from Geometry import Point p = Point(5,7) from Geometry import Point p = Point(5,7) Use the class like this p x:5 y:7 The object in memory Lecture03COMPSCI 1056 Geometry.py

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) from Geometry import Point p = Point(5, 7) Lecture03COMPSCI 1057

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 parameters class Point: def __init__(self, loc_x, loc_y): self.x = loc_x self.y = loc_y class 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) from Geometry import Point p = Point(5, 7) print(p.x) print(p.y) Lecture03COMPSCI 1058 Saved in a file called Geometry.py

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 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 Lecture03COMPSCI 1059

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 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 Point p = Point(0,0) p.translate(10,5) from Geometry import Point p = Point(0,0) p.translate(10,5) Lecture03COMPSCI 10510

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 Square side = 10 s = Square(side) from Geometry import Square side = 10 s = Square(side) print (s.perimeter()) Lecture03COMPSCI

 Write a class to represent fractions in Python  create a fraction  add  subtract  multiply  divide  text representation Example: Fractions ½ numerator denominator Lecture03COMPSCI 10512

Model of objects in memory methods state num: den: 7 8 methods state num: den: 3 4 methods state num: den: 1 2 x y z from Fraction import Fraction x = Fraction(1,2) y = Fraction(3,4) z = Fraction(7,8) from Fraction import Fraction x = Fraction(1,2) y = Fraction(3,4) z = Fraction(7,8) Lecture03COMPSCI Example03.py

 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 class Fraction: def __init__(self, top, bottom): self.num = top #numerator self.den = bottom #denominator Lecture03COMPSCI 10514

 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.num 3 >>> x.den 4 >>> x.num 3 >>> x.den 4 >>> x = Fraction(3, 4) Lecture03COMPSCI Lecture 04

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__ Lecture03COMPSCI 10516

 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?' name = 'Andrew' greeting = 'Hello ' + name + '. How are you?' my_name = 'Andrew' greeting = 'Hello {name}. How are you?'.format(name=my_name) 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) first = 'Andrew' second = 'Luxton-Reilly' greeting = 'Hello {0} {1}'.format(first, second) Lecture03COMPSCI 10517

 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) 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 >>> x = Fraction(2, 3) >>> x Fraction(2, 3) Lecture03COMPSCI 10518

Without the __repr__ method >>> x = Fraction(2, 3) >>> x >>> x = Fraction(2, 3) >>> x class Fraction: def __init__(self, top, bottom): self.num = top self.den = bottom class Fraction: def __init__(self, top, bottom): self.num = top self.den = bottom ) Lecture03COMPSCI 10519

 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) def __str__(self): return str(self.num) + '/' + str(self.den) >>> x = Fraction(3, 4) >>> print(x) 3/4 >>> x = Fraction(3, 4) >>> print(x) 3/4 Lecture03COMPSCI 10520

Without the __str__ method >>> x = Fraction(2, 3) >>> print(x) >>> x = Fraction(2, 3) >>> print(x) class Fraction: def __init__(self, top, bottom): self.num = top self.den = bottom class Fraction: def __init__(self, top, bottom): self.num = top self.den = bottom Lecture03COMPSCI 10521

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? Lecture03COMPSCI 10522