COMP 204. Object Oriented Programming (OOP) - Inheritance. Mathieu Blanchette

Transcription

1 COMP 204 Object Oriented Programming (OOP) - Inheritance Mathieu Blanchette 1 / 14

2 Inheritance Motivation: We often need to create classes that are closely related but not identical to an existing class. Example: We already have created a Bus class with attributes: station, capacity, passengers, terminus methods: init, move, unload, load, str To represent a bus where passengers have to pay to board, we may want to add new attributes like the price of the ticket and the total amount of money present on the bus. To represent an express bus that only stops at certain stops, we may want to add attributes about the stops the bus will make, and modify the load/unload methods accordingly. Note: We want to continue to use all the other attributes and methods defined on the Bus class. 2 / 14

3 Inheritance How can we do this? A bad approach: Duplication Create a PayBus class. Copy-paste the Bus class code into it. Add a new attribute cost of ticket and cash onboard. Bad because: We now have two copies of the Bus code. If we want to make a change to the Bus class (e.g. bug fix, or improvement), we have to remember to make the same change to the PayBus class. Makes program large, difficult to understand. Good approach: Inheritance Create a PayBus class that inherits the attributes and methods of the Patient class. 3 / 14

4 The Bus class 1 c l a s s Bus : 2 d e f i n i t ( s e l f ) : 3 s e l f. s t a t i o n = 0 # t h e p o s i t i o n o f t h e bus 4 s e l f. c a p a c i t y = 5 # t h e c a p a c i t y o f t h e bus 5 s e l f. p a s s e n g e r s = [ ] # t h e c o n t e n t o f t h e bus 6 s e l f. t e r m i n u s = 5 # The l a s t s t a t i o n 7 8 d e f move ( s e l f ) : 9 # code not shown d e f u n l o a d ( s e l f ) : 12 # code not shown d e f l o a d ( s e l f, w a i t i n g l i n e ) : 15 # code not shown d e f s t r ( s e l f ) : 18 # code not shown 4 / 14

5 Creating a subclass To create a new class PayBus that is a subclass of the Bus class: 1 c l a s s PayBus ( Bus ) : 2 d e f i n i t ( s e l f, p r i c e =2) : 3 Bus. i n i t ( s e l f ) 4 s e l f. c o s t o f t i c k e t = p r i c e # c o s t o f a t i c k e t 5 s e l f. cash = 0 # t h e t o t a l cash onboard Notes: The PayBus class is a subclass of Bus because of line 1: class PayBus(Bus): PayBus inherits the attributes and methods of the Bus class. Those get initialized by the line: Bus. init (self) which calls the init method of the parent class. Note: Since we are calling the method directly on the class rather than on an object, self needs to be explicitly passed as an argument. PayBus extends the Bus class by adding two new attributes: cost of ticket and cash 5 / 14

6 PayBus class The PayBus class has 6 attributes (station, capacity, passengers, terminus, cost of ticket, cash) and 5 methods ( init, move, unload, load, str ). They can be used like with any other class: 1 stm bus = PayBus ( 2 ) 2 stm bus. l o a d ( [ 3, 4, 5, 2, 6, 2, 3 ] ) 3 stm bus. s t a t i o n = 3 4 stm bus. cash = p r i n t ( stm bus ) 6 # P r i n t s : Bus a t s t a t i o n 3 c o n t a i n s p a s s e n g e r s [ 3, 4, 5, 2, 6 ]. OK, so our PayBus class has this extra attribute cash, but the Bus methods don t know about it... 6 / 14

7 Overriding the load method Goal: Make new passengers pay price of ticket when they board, add to cash Approach: Create a new load() method in the PayBus class. It will override the load() method of the parent class. 1 c l a s s PayBus ( Bus ) : 2 d e f i n i t ( s e l f, p r i c e =2) : 3 Bus. i n i t ( s e l f ) 4 s e l f. c o s t o f t i c k e t = p r i c e # c o s t o f a t i c k e t 5 s e l f. cash = 0 # t h e t o t a l cash onboard 6 7 d e f l o a d ( s e l f, w a i t i n g l i n e ) : 8 n u m b e r b o a r d i n g = Bus. l o a d ( s e l f, w a i t i n g l i n e ) 9 s e l f. cash += n u m b e r b o a r d i n g s e l f. c o s t o f t i c k e t 10 r e t u r n n u m b e r b o a r d i n g Notes: The load() method first calls the load method of the parent class. This deals with adding new passengers. It then updates the amount of cash on the bus. 1 stm bus = PayBus ( 2 ) 2 stm bus. l o a d ( [ 3, 4, 5, 2, 6, 2, 3 ] ) 3 p r i n t ( Cash =, stm bus. cash ) # P r i n t s Cash = 10 7 / 14

8 Overriding the str method We should also revise the str method to make it print information about the amount of cash on board. 1 c l a s s PayBus ( Bus ) : 2 d e f i n i t ( s e l f, p r i c e =2) : 3 Bus. i n i t ( s e l f ) 4 s e l f. c o s t o f t i c k e t = p r i c e # c o s t o f a t i c k e t 5 s e l f. cash = 0 # t h e t o t a l cash onboard 6 7 d e f l o a d ( s e l f, w a i t i n g l i n e ) : 8 n u m b e r b o a r d i n g = Bus. l o a d ( s e l f, w a i t i n g l i n e ) 9 s e l f. cash += n u m b e r b o a r d i n g s e l f. c o s t o f t i c k e t 10 r e t u r n n u m b e r b o a r d i n g d e f s t r ( s e l f ) : 13 r e t u r n Bus. s t r ( s e l f )+\ 14. Amount o f cash : + s t r ( s e l f. cash ) 8 / 14

9 ExpressBus class A class like Bus can have many different subclasses. We will create an ExpressBus subclass. An express bus differs from a normal bus in that it only stops at certain predetermined stop. 1 c l a s s E x p r e s s B u s ( Bus ) : 2 d e f i n i t ( s e l f, my stops ) : 3 Bus. i n i t ( s e l f ) 4 s e l f. s t o p s = my stops # l i s t o f s t a t i o n s 5 # where t h e bus w i l l s t o p Note: We could also have decided that the ExpressBus class is a subclass of the PayBus class, if we needed the functionality of payments. 9 / 14

10 ExpressBus class We now need to override the load and unload methods to only allow boarding/unloading if we are at a station where the bus stops. 1 c l a s s E x p r e s s B u s ( Bus ) : 2 d e f i n i t ( s e l f, my stops ) : 3 Bus. i n i t ( s e l f ) 4 s e l f. s t o p s = my stops # l i s t o f s t a t i o n s 5 # where t h e bus w i l l s t o p 6 7 d e f u n l o a d ( s e l f ) : 8 i f s e l f. s t a t i o n i n s e l f. s t o p s : 9 r e t u r n Bus. u n l o a d ( s e l f ) # a l l o w u n l o a d i n g 10 e l s e : 11 r e t u r n [ ] # no u n l o a d i n g d e f l o a d ( s e l f, w a i t i n g l i n e ) : 14 i f s e l f. s t a t i o n i n s e l f. s t o p s : # a l l o w l o a d i n g 15 r e t u r n Bus. l o a d ( s e l f, w a i t i n g l i n e ) 16 e l s e : # no l o a d i n g 17 r e t u r n 0 10 / 14

11 ExpressBus class See the difference between the Bus and ExpressBus classes: 1 exp = E x p r e s s B u s ( [ 0, 2, 4 ] ) # bus w i l l s t o p o n l y a t 0, 2, 4 2 s l o w = Bus ( ) 3 exp. l o a d ( [ 5, 3, 1 ] ) 4 s l o w. l o a d ( [ 5, 3, 1 ] ) 5 p r i n t ( exp ) # Bus a t s t a t i o n 0 c o n t a i n s p a s s e n g e r s [ 5, 3, 1 ] 6 p r i n t ( s l o w ) # Bus a t s t a t i o n 0 c o n t a i n s p a s s e n g e r s [ 5, 3, 1 ] 7 8 exp. move ( ) 9 s l o w. move ( ) 10 exp. l o a d ( [ 4, 3 ] ) # Nobody g e t s l o a d e d onto e x p r e s s bus 11 s l o w. l o a d ( [ 4, 3 ] ) # But p a s s e n g e r s can board t h e s l o w bus 12 p r i n t ( exp ) # Bus a t s t a t i o n 1 c o n t a i n s p a s s e n g e r s [ 5, 3, 1 ] 13 p r i n t ( s l o w ) # Bus a t s t a t i o n 1 c o n t a i n s p a s s e n g e r s [ 5, 3, 1, 4, 3 ] 11 / 14

12 ExpressBus class Subclasses can also define their own methods. Example: Load safe() method only allows boarding for people whose destination is among the stops the express buss will make. 1 d e f l o a d s a f e ( s e l f, w a i t i n g l i n e ) : 2 # o n l y a l l o w s b o a r d i n g f o r p a s s e n g e r s 3 # whose d e s t i n a t i o n i s one o f t h e s t o p s 4 # o f t h e e x p r e s s bus 5 s h o u l d b o a r d = [ p f o r p i n w a i t i n g l i n e \ 6 i f p i n s e l f. s t o p s ] 7 n u m b e r b o a r d i n g = min ( l e n ( s h o u l d b o a r d ),\ 8 s e l f. c a p a c i t y l e n ( s e l f. p a s s e n g e r s ) ) 9 p e o p l e b o a r d i n g = s h o u l d b o a r d [ 0 : n u m b e r b o a r d i n g ] 1 exp = E x p r e s s B u s ( [ 0, 2, 4 ] ) 2 exp. l o a d s a f e ( [ 4, 2, 3, 1, 3, 2 ] ) 3 p r i n t ( exp ) # Bus a t s t a t i o n 0 c o n t a i n s p a s s e n g e r s [ 4, 2, 2 ] 4 5 s l o w = Bus ( ) 6 s l o w. l o a d s a f e ( [ 4, 2, 3, 1, 3, 2 ] ) 7 # A t t r i b u t e E r r o r : Bus o b j e c t has no a t t r i b u t e l o a d s a f e 12 / 14

13 Extending BioPython classes We can write new classes that extend any existing class, including those defined in BioPython. Example: Define the MySeq class that extends the Seq class to add a list of confidence values (between 0 and 1) associated to each character in the sequence an average confidence() method that computes the average confidence values for the sequence a gc content() method that computes the fraction of bases that are either G or C 13 / 14

14 Extending BioPython classes 1 from Bio. Seq i m p o r t Seq 2 3 c l a s s MySeq ( Seq ) : 4 d e f i n i t ( s e l f, seq, c o n f ) : 5 Seq. i n i t ( s e l f, seq ) 6 s e l f. c o n f i d e n c e = c o n f # c o n f i d e n c e v a l u e s 7 8 # Seq doesn t compute GC c o n t e n t so 9 # we l l add t h a t f u n c t i o n a l i t y 10 d e f g c c o n t e n t ( s e l f ) : 11 r e t u r n l e n ( [ b f o r b i n s e l f i f b i n GC ] ) / l e n ( s e l f ) d e f a v g c o n f i d e n c e ( s e l f ) : 14 r e t u r n sum ( s e l f. c o n f i d e n c e ) / l e n ( s e l f. c o n f i d e n c e ) seq1 = Seq ( ACGTATG ) 17 seq2 = MySeq ( AAACG, [ 0. 9, 0. 8, 0. 5, 1, 0. 8 ] ) 18 p r i n t ( GC c o n t e n t =, seq2. g c c o n t e n t ( ) ) 19 p r i n t ( Average c o n f i d e n c e v a l u e =, seq2. a v g c o n f i d e n c e ( ) ) 14 / 14