Object Oriented Programming Revisited

1. Object Oriented Programming Revisited • Classes • Ideal for modeling real-world phenomena • Encapsulate data: Attributes • Encapsulate function: Behaviors • Act as blueprints • Objects are instantiated from classes

2. An abstract game with two players • Player: • Name • Overall skill (5.0 – 10.0) • Motivation flux mf (0.0 – 1.0) • Current strength: • Current motivation cm : random number in [ -mf, mf ] • Current strength = skill + skill * cm With motivation 1.0, current strength is 2 * skill. With motivation 0.0, current strength is skill. Motivation flux 0.0: very stable player. Motivation flux 1.0: very unstable player.

3. player.py • Object reference self first argument in all methods • self refers to the object on which the method is called • Object attribute self.skill vs. local variable skill

4. No self argument in method calls! Python puts object reference in place of self >>> from player import Player >>> john = Player('John', 1.0, 8.0) >>> paul = Player('Paul') >>> print paul.skill 7.5 john paul name = ‘John’ skill = 8.0 motivation_flux = 1.0 name = ‘Paul’ skill = 7.5 motivation_flux = 0.0 class Player

5. Summary • Class header • Keyword class begins definition, followed by name of class and colon (:) • Body of class • Indented block of code • Optional documentation string • Describes the class • Appears immediately after class header • Constructor method __init__ • Executes each time an object is created • Initialize attributes of class • Returns None • Object reference • All methods must at least specify this one parameter • Represents object of class on which a method is called • Called self by convention

6. Remember the self. prefix when referencing object attributes >>> from player_wrong import Player >>> ringo = Player("Ringo") >>> print ringo.my_skill Traceback (most recent call last): File "<stdin>", line 1, in ? AttributeError: Player instance has no attribute ‘my_skill'

7. Let’s do something with our objects: method play player2.py >>> from player2 import Player >>> john = Player("John", 1.0, 8.0) >>> paul = Player("Paul") >>> john.play(paul) John (11.97) - Paul (7.50): John wins >>> john.play(paul) John (1.88) - Paul (7.50): Paul wins >>> paul.play(john) Paul (7.50) - John (3.32): Paul wins

8. Destructors • Constructor named __init__ • Destructor • Method is named __del__ • Executed automatically when object is destroyed or when no more references to object exist • Can be used for housekeeping before Python reclaims object memory

9. Class Attributes • Class Attributes • One copy of attribute shared by all objects of a class • Represents “class-wide” information • Property of the class, not of an object of the class • Initialized once in the class definition (not in constructor) • Otherwise it would be re-initialized every time an object is created • Accessed through the class name or any object of the class • May exist even if no objects of class exist

10. Adding class attribute count + destructor player3.py >>> from player3 import Player >>> paul = Player('Paul') >>> ringo = Player('Ringo') >>> del paul Player Paul deleted

11. Preparing to hold tournaments • New object attribute winsto keep track of match results • New play method which updates attribute wins player4.py

12. Testing.. >>> from player4 import Player >>> john = Player('John', 0.2, 8.0) >>> paul = Player('Paul') >>> john.play(paul) John defeats Paul >>> john.play(paul) John defeats Paul >>> john.play(paul) John defeats Paul >>> john.play(paul) Paul defeats John >>> print john.wins 3 >>> print paul.wins 1 >>> ^D Player Paul deleted Player John deleted threonine:~/python% NB: destructor called automatically when session ends

13. Now we’re ready to hold a tournament! player_game.py

14. Holding tournament with four players threonine:~/python% python player_game.py Player name: John John's skill (5.0-10.0): 8 John's motivation flux (0.0-1.0): 0.2 Player name: Paul Paul's skill (5.0-10.0): 7.5 Paul's motivation flux (0.0-1.0): 0 Player name: George George's skill (5.0-10.0): 7.5 George's motivation flux (0.0-1.0): 0.5 Player name: Ringo Ringo's skill (5.0-10.0): 7.5 Ringo's motivation flux (0.0-1.0): 1.0 Paul defeats John John defeats George Ringo defeats John George defeats Paul Ringo defeats Paul Ringo defeats George Ringo is the winner with 3 wins! Player initialization Tournament matches