COMPSCI 107 Computer Science Fundamentals

1. COMPSCI 107Computer Science Fundamentals Lecture 08 – Classes

2. Recap • Exceptions alter the flow of control • When an exception is raised, execution stops • When the exception is caught, execution starts again • try… except blocks are used to handle problem code • Can ensure that code fails gracefully • Can ensure input is acceptable • raise • Creates a runtime exception • finally • Executes code after the exception handling code COMPSCI 107 - Computer Science Fundamentals

3. Learning outcomes • At the end of this lecture, students should be able to: • Define a new class • Store state information about instances of the class • Define new methods of the class • Override the default behaviour for standard operations COMPSCI 107 - Computer Science Fundamentals

4. 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 COMPSCI 107 - Computer Science Fundamentals

5. Example: Point class • Defining and using a simple class class Point: def __init__(self, loc_x, loc_y): self.x = loc_x self.y = loc_y >>> origin = Point(0, 0) >>> destination = Point(34, 65) >>> destination.x 34 >>> destination.y 65 COMPSCI 107 - Computer Science Fundamentals

6. Example: Fractions • Write a class to represent fractions in Python • create a fraction • add • subtract • multiply • divide • text representation ½ numerator denominator COMPSCI 107 - Computer Science Fundamentals

7. Model of objects in memory state state state x 1 7 3 num: num: num: z 4 8 2 den: den: den: y methods methods methods COMPSCI 107 - Computer Science Fundamentals

8. Constructor • All classes must have a constructor • constructor is a special function that creates the object • always called __init__ • the first argument is always called self • self • a reference to the object that has been created • used to access the state variables and methods of the object class Fraction: def __init__(self, top, bottom): self.num = top #numerator self.den = bottom #denominator COMPSCI 107 - Computer Science Fundamentals

9. Using the Fraction class • 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! >>> x = Fraction(3, 4) >>> x.num 3 >>> x.den 4 COMPSCI 107 - Computer Science Fundamentals

10. 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 COMPSCI 107 - Computer Science Fundamentals

11. __repr__ • The __repr__ method produces an string showing an unambiguous way to represent the object (i.e. the "official" representation) • 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)' >>> x = Fraction(2, 3) >>> x <__main__.Fraction object at 0x02762290> def __repr__(self): return 'Fraction(%i, %i)' % (self.num, self.den) >>> x = Fraction(2, 3) >>> x Fraction(2, 3) COMPSCI 107 - Computer Science Fundamentals

12. __str__ • The __str__ method returns a string representing the object • Default behaviour is to use the __repr__ function (if not implemented, gives us memory) • We should implement a version with a natural representation: • After we have implemented the method, we can use standard Python >>> x = Fraction(3, 4) >>> print(x) <__main__.Fraction object at 0x02714290> def __str__(self): return str(self.num) + '/' + str(self.den) >>> x = Fraction(3, 4) >>> print(x) 3/4 COMPSCI 107 - Computer Science Fundamentals

13. __add__ • The __add__ method is called when the + operator is used • If we implement __add__ then we can use + to add the objects • f1 + f2 gets translated into f1.__add__(f2) def __add__(self, other): new_num = (self.num * other.den + self.den * other.num) new_den = self.den * other.den return Fraction(new_num, new_den) x = Fraction(1, 2) y = Fraction(1, 4) z = x + y print(z) 6/8 COMPSCI 107 - Computer Science Fundamentals

14. Greatest Common Divisor • Use Euclid's Algorithm • Given two numbers, n and m, find the number k, such that k is the largest number that evenly divides both n and m. defgcd(m, n): while m % n != 0: old_m = m old_n = n m = old_n n = old_m % old_n return n COMPSCI 107 - Computer Science Fundamentals

15. Improve the add method • We can improve the add operation to always represent a fraction using the "lowest terms" form. • What other things might we want to add to a Fraction? def__add__(self, other): new_num= self.num* other.den + self.den* other.num new_den= self.den * other.den common = gcd(top, bottom) returnFraction(new_num // common, new_den // common) COMPSCI 107 - Computer Science Fundamentals

16. __eq__ • The __eq__ method checks equality of the objects • Default behaviouris to compare the references • We want to compare the contents def __eq__(self, other): return self.num* other.den == other.num* self.den COMPSCI 107 - Computer Science Fundamentals

17. Exercise • What is the output of the following code? x = Fraction(2, 3) y = Fraction(1, 3) z = y + y print(x == z) print(x is z) w = x + y print(w == 1) COMPSCI 107 - Computer Science Fundamentals

18. Improving __eq__ • Check the type of the other operand • If the type is not a Fraction, then not equal? • What other decisions could we make for equality? def __eq__(self, other): if notisinstance(other, Fraction): return False returnself.num * other.den == other.num * self.den COMPSCI 107 - Computer Science Fundamentals

19. Other standard Python operators • object.__lt__(self, other) • object.__le__(self, other) • object.__eq__(self, other) • object.__ne__(self, other) • object.__gt__(self, other) • object.__ge__(self, other) • object.__sub__(self, other) • object.__mul__(self, other) • object.__truediv__(self, other) • object.__iadd__(self, other) COMPSCI 107 - Computer Science Fundamentals

20. Summary Classes are defined using the syntax: Classes contain methods that define the behaviour and operations on objects of that class We override the default behaviours for built-in methods/operations Additional Resources http://interactivepython.org/runestone/static/thinkcspy/Classes/fractions.html class name_of_the_class: definition of the class goes here COMPSCI 107 - Computer Science Fundamentals