1. Classes 2 COMPSCI 105 S2 2015 Principles of Computer Science

2. Exercise  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()) Lecture05COMPSCI 1052 40

3.  Write a class to represent fractions in Python  create a fraction  add  subtract  multiply  divide  text representation Example: Fractions ½ numerator denominator Lecture05COMPSCI 1053

4. 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) Lecture05COMPSCI 1054

5.  All classes must have a constructor  The constructor for a Fraction should store the numerator and the denominator  So far, we can create a Fraction 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 Lecture05COMPSCI 1055 >>> x = Fraction(3, 4) >>> x.num 3 >>> x.den 4 >>> x = Fraction(3, 4) >>> x.num 3 >>> x.den 4

6.  We can access and changing the state variables directly  Direct access of a data field in an object is not a good practice.  The data may be tempered with, e.g., the denominator should never be 0, but it may be mistakenly set.  The class becomes difficult to maintain and vulnerable to bugs, e.g., a change to the type of a data field in the class effects other program that uses the class. Using the Fraction class >>> x.num 3 >>> x.den -= 4 >>> x.den 0 >>> x.num 3 >>> x.den -= 4 >>> x.den 0 Lecture05COMPSCI 1056

7.  To prevent direct modification of data fields, don’t let the client (user) directly access the data fields of an object.  This is known as data hiding, which can be done by defining private data fields in a class.  In Python, the private data fields are defined with two leading underscores.  You can also define a private method named with two leading underscores. Hiding the data fields Lecture05COMPSCI 1057 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

8.  Private data fields (or methods) can be accessed within in a class, but they can not be accessed outside the class.  To make a private data field accessible for the client, provide a get method to return its value.  To enable a private data field to be modifiable, provide a set method to set a new value. Accessing private data fields Lecture05COMPSCI 1058 >>> x= Fraction(3,4) >>> x.__num Traceback (most recent call last): File " ", line 1, in AttributeError: 'Fraction' object has no attribute '__num' >>> >>> x= Fraction(3,4) >>> x.__num Traceback (most recent call last): File " ", line 1, in AttributeError: 'Fraction' object has no attribute '__num' >>>

9.  A get method is referred to as an accessor method.  A set method is referred to as a mutator method.  Accessing the private data fields through accessor and mutator methods. Accessor and mutator methods Lecture05COMPSCI 1059 def getNumerator(self): return self.__num def getNumerator(self): return self.__num def SetNumerator(self, top): self.__num = top def SetNumerator(self, top): self.__num = top >>> x = Fraction(3,4) >>> x.getNumerator() 3 >>> x.setNumerator(10) >>> x.getNumerator() 10 >>> x = Fraction(3,4) >>> x.getNumerator() 3 >>> x.setNumerator(10) >>> x.getNumerator() 10

10. Overriding default behaviour  All classes have a number of methods provided by default, e.g.  methods to do with representing the output format of the object  methods to do with comparing two objects  methods to do with computation between objects, ……  Since default behaviour is not always very useful and accurate, we should write our own versions of those methods. Lecture05COMPSCI 10510 >>> x >>> y = Fraction(1,4) >>> print(y) >>> z = Fraction(1,4) >>> y == z False >>> x >>> y = Fraction(1,4) >>> print(y) >>> z = Fraction(1,4) >>> y == z False

11.  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’ last = 'Luxton-Reilly' greeting = 'Hello {0} {1}!'.format(first, last) first = 'Andrew’ last = 'Luxton-Reilly' greeting = 'Hello {0} {1}!'.format(first, last) Lecture05COMPSCI 10511

12.  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 The __repr__ method 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) Lecture05COMPSCI 10512

13.  With the __repr__ method defined in the class, it produces: 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 Lecture05COMPSCI 10513 def __repr__(self): return 'Fraction({0}, {1})'.format(self.__num, self.__den) def __repr__(self): return 'Fraction({0}, {1})'.format(self.__num, self.__den) >>> x = Fraction(2, 3) >>> x >>> Fraction(2, 3) >>> x = Fraction(2, 3) >>> x >>> Fraction(2, 3)

14.  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 The __str__ method 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 Lecture05COMPSCI 10514

15.  With the __str__ method defined in the class, it produces: 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 Lecture05COMPSCI 10515 def __str__(self): return str(self.__num) + '/' + str(self.__den) def __str__(self): return str(self.__num) + '/' + str(self.__den) >>> x = Fraction(2, 3) >>> print(x) >>> 2/3 >>> x = Fraction(2, 3) >>> print(x) >>> 2/3

16. Exercise  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? Lecture05COMPSCI 10516