Object-Oriented Programming

Wednesday, 7/15

Announcements

• Project 2 due Thursday
• Homework 5 due Friday
• Midterm 1 regrade requests due Sunday
• We regraded Q4b (intersection) with a new rubric
• Please fill out the mid-semester survey. We appreciate any feedback you can give us!
• "Writing code live is good, but having code on lecture slides is good for reference"
• "Labs and discussions should have trickier problems"
• "Albert, 1v1 me in [Super Smash Bros]"
• PSA: the full set of lecture slides are posted after lecture. Be sure to check the website after class

Object-Oriented Programming

1. Review
2. Object-Oriented Programming
1. Classes and objects
2. Attributes
3. Methods
4. Property methods
5. Private variables (sort of)

Review: Functions

• Functions: processes that describe computations
• Order of evaluation
• Environments
• Control structures
• Recursion

Functions

describing computations

Review: Data

• Data: structures that store information
• Lists, dictionaries, strings
• Data abstraction
• Linked lists: sequential data
• Trees: hierarchical data

Functions

describing computations

Data

Storing information

Review: State

• State: changing information over time
• Mutabililty: can change state
• Lists, dictionaries
• Immutability: cannot change state
• Tuples, strings
• Nonlocal state in environments

Functions

describing computations

Data

Storing information

State

Changing information over time

Object-Oriented Programming

• Review
• Object-Oriented Programming
• Classes and objects
• Attributes
• Methods
• Property methods
• Private variables (sort of)

Life with data abstraction

def rational(num, denom):

return [num, denom]

​

def numer(r):

return r[0]

​

def denom(r):

return r[1]

​

• Abstract data type: defined in terms of behavior
• Abstraction: supposed to treat it as a "rational" object
• Abstraction violation: it's tempting to say "rational just returns a list!"

Life with data abstraction

Data abstraction

Can we do better?

Good idea:

easier to read and maintain code

Consequences:

prone to abstraction violations

Object-Oriented Programming

• Review
• Object-Oriented Programming
• Classes and objects
• Attributes
• Methods
• Property methods
• Private variables (sort of)

Object-Oriented Programming

• Object-oriented programming (OOP): a programming paradigm (technique)
• Think of code as abstract objects
• Objects have attributes
• Objects can do actions
• Different types of objects interact with each other

Name: Albert

Favorite food: burgers

walk to a destination

pet an animal

Name: Sally

Favorite food: fries

climb a tree

pose for pictures

Object-Oriented Programming

• Every object is an instance of a class
• class: a type or a category of objects

Albert is a human

Sally is a squirrel

object

class

Object-Oriented Programming

• Review
• Object-Oriented Programming
• Classes and objects
• Attributes
• Methods
• Property methods
• Private variables (sort of)

Object-Oriented Programming: learning by example

• Suppose we're building a Pokemon game
• Pokemon are creatures with different types and abilities
• People train pokemon and battle each other

Acknowledgements: Tom Magrino and Jon Kotker developed the original Pokemon example for CS 61A, Summer 2012

Classes and objects

class Pokemon:

...

• A class defines a blueprint for all objects of that type
• The class keyword creates a class
• Pokemon is the name of this class

"Here's what a Pokemon looks like!"

Classes and objects

class Pokemon:

def __init__(self, name):

...

• The __init__ method creates an object (an instance of the class)
• Called a constructor
• A method is like a function (more on this later)
• To construct a Pokemon, we give it a name (parameters)

"I want to create my own Pokemon!"

Classes and objects

class Pokemon:

def __init__(self, name):

self.name = name

self.level = 1

self.hp = 5 * self.level

self.damage = 2 * self.level

• Objects can have instance attributes
• Used to store state for a particular object

"When I create my Pokemon, I'll give it a name and it'll start at level 1"

self.level might change later!

Classes and objects

class Pokemon:

def __init__(self, name):

self.name = name

self.level = 1

self.hp = 5 * self.level

self.damage = 2 * self.level

(video demo)

>>> ashs_pikachu = Pokemon('pikachu')

>>> ashs_pikachu

<Pokemon object at …>

Object-Oriented Programming

• Review
• Object-Oriented Programming
• Classes and objects
• Attributes
• Methods
• Property methods
• Private variables (sort of)

Attributes: Dot notation

>>> ashs_pikachu = Pokemon('pikachu')

>>> ashs_pikachu.name

'pikachu'

>>> ashs_pikachu.level

1

>>> ashs_pikachu.hp

5

ashs_pikachu.name

object

attribute

"The name of ashs_pikachu"

Attributes: Instance attributes

>>> ashs_pikachu = Pokemon('pikachu')

>>> ashs_pikachu.name

'pikachu'

​

>>> alberts_jigglypuff = Pokemon('jigglypuff')

>>> alberts_jigglypuff.name

'jigglypuff'

• Different objects of same class maintain their own copies of instance attributes

"The name of ashs_pikachu is different from the name of alberts_jigglypuff"

Attributes: self

class Pokemon:

def __init__(self, name):

self.name = name

self.level = 1

self.hp = 5 * self.level

self.damage = 2 * self.level

self is a variable that is bound to an instance of this class

From Pikachu's point of view:

"My name will be 'pikachu', and I will start at level 1"

Attributes: State

• Attributes can be reassigned
• State changes over time (mutation)

>>> ashs_pikachu = Pokemon('pikachu')

>>> ashs_pikachu.level

1

​

>>> ashs_pikachu.level += 9000

>>> ashs_pikachu.level

9001

"ashs_pikachu's leveled up!"

Attributes: State

• Attributes can be dynamically assigned
• Not just when object is created

>>> ashs_pikachu = Pokemon('pikachu')

>>> ashs_pikachu.status = 'confused'

>>> ashs_pikachu.status

'confused'

"ashs_pikachu's is confused!"

Attributes: Class attributes

class Pokemon:

population = 0

def __init__(self, name):

self.name = name

self.level = 1

self.hp = 5 * self.level

self.damage = 2 * self.level

Pokemon.population += 1

>>> Pokemon.population

0

>>> ashs_pikachu = Pokemon('pikachu')

>>> Pokemon.population

1

>>> alberts_jigglypuff = Pokemon('jigglypuff')

>>> Pokemon.population

2

Attributes: Attribute lookup

>>> ashs_pikachu = Pokemon('pikachu')

>>> Pokemon.population

1

>>> ashs_pikachu.population

1

Pokemon

​

population: 0

Pokemon

​

population: 1

Attributes: Attribute lookup

>>> ashs_pikachu = Pokemon('pikachu')

>>> Pokemon.population += 150

>>> ashs_pikachu.population

151

Pokemon

​

population: 1

Pokemon

​

population: 151

Attributes: Attribute lookup

>>> ashs_pikachu = Pokemon('pikachu')

>>> Pokemon.population += 150

>>> ashs_pikachu.population -= 126

>>> ashs_pikachu.population

25

>>> Pokemon.population

151

ashs_pikachu

​

name: 'pikachu'

level: 1

hp: 5

damage: 2

population: 25

Pokemon

​

population: 151

ashs_pikachu.population = ashs_pikachu.population - 126

Evaluated first: 151

Assigned to 25

Object identity and type checking

>>> ashs_pikachu = Pokemon('pikachu')

>>> alberts_jigglypuff = Pokemon('jigglypuff')

>>> ashs_pikachu is alberts_jigglypuff

False

>>> roberts_pikachu = ashs_pikachu

>>> ashs_pikahu is roberts_pikachu

True

>>> roberts_pikachu = Pokemon('pikachu')

>>> ashs_pikachu is roberts_pikachu

False

​

>>> isinstance(ashs_pikachu, Pokemon)

True

>>> type(ashs_pikachu)

<class 'Pokemon'>

(demo)

Object-Oriented Programming

• Review
• Object-Oriented Programming
• Classes and objects
• Attributes
• Methods
• Property methods
• Private variables (sort of)

Methods: Your turn!

class Pokemon:

def __init__(self, name):

self.name = name

self.level = 1

self.hp = 5 * self.level

self.damage = 2 * self.level

​

def talk(self):

print(self.name + '!')

• Method: a function that is bound to an object
• Usually describes an action that the object can do
• self refers to the object that the method is bound to

"When a Pokemon talks, it prints its name excitedly!"

self is just a variable. We can technically call it anything. It is called self by convention

Methods: Usage

class Pokemon:

...

def talk(self):

print(self.name + '!')

>>> ashs_pikachu = Pokemon('pikachu')

>>> ashs_pikachu.talk()

pikachu!

>>> ashs_pikachu.talk(ashs_pikachu)

TypeError

Methods: Methods vs. Functions

class Pokemon:

...

def talk(self):

print(self.name + '!')

>>> ashs_pikachu = Pokemon('pikachu')

>>> ashs_pikachu.talk()

pikachu!

>>> Pokemon.talk(ashs_pikachu)

pikachu!

Method call: bound to an object

Function call: not bound

(demo)

Python distinguishes between

• functions (we've been writing these for past few weeks)
• methods (bound to objects)

Methods: Your turn!

class Pokemon:

...

def decrease_hp(self, amount):

"*** YOUR CODE HERE ***"

​

def attack(self, other):

"*** YOUR CODE HERE ***"

• Implement decrease_hp:
• decreases hp by amount
• hp cannot go below 0
• Implement attack:
• prints "<name> attacks <other.name>"
• decreases other Pokemon's hp by self.damage

Methods

class Pokemon:

...

def decrease_hp(self, amount):

self.hp -= amount

if self.hp < 0:

self.hp = 0

​

def attack(self, other):

print(self.name,

'attacks',

other.name)

other.decrease_hp(self.damage)

• Implement decrease_hp:
• decreases hp by amount
• hp cannot go below 0
• Implement attack:
• prints "<name> attacks <other.name>"
• decreases other Pokemon's hp by self.damage

Object-Oriented Programming

• Review
• Object-Oriented Programming
• Classes and objects
• Attributes
• Methods
• Property methods
• Private variables (sort of)

Property methods

class Pokemon:

def __init__(self, name):

self.name = name

self.level = 1

self.hp = 5 * self.level

self.damage = 2 * self.level

​

def level_up(self):

self.level += 1

self.hp = 5 * self.level

self.damage = 2 * self.level

>>> ashs_pikachu.level_up()

>>> ashs_pikachu.level

2

>>> ashs_pikachu.hp

10

>>> ashs_pikachu.level = 5

>>> ashs_pikachu.hp

10

Have to duplicate code for assigning hp and damage

Property methods

class Pokemon:

def __init__(self, name):

self.name = name

self.level = 1

​

def level_up(self):

self.level += 1

​

@property

def hp(self):

return 5 * self.level

​

@property

def damage(self):

return 2 * self.level

>>> ashs_pikachu.level_up()

>>> ashs_pikachu.level

2

>>> ashs_pikachu.hp

10

>>> ashs_pikachu.level = 5

>>> ashs_pikachu.hp

25

Property method:

technically a method, but acts like an attribute

No parentheses!

Property methods

>>> ashs_pikachu.level_up()

>>> ashs_pikachu.level

2

>>> ashs_pikachu.hp = 9001

AttributeError

>>> ashs_pikachu.level_up()

>>> ashs_pikachu.level

2

>>> ashs_pikachu.hp = 9001

>>> ashs_pikachu.hp

9001

Can't assign to a property method

Instance attribute

Property method

Object-Oriented Programming

• Review
• Object-Oriented Programming
• Classes and objects
• Attributes
• Methods
• Property methods
• Private variables (sort of)

Private variables (sort of)

class Pokemon:

def __init__(self, name):

self.name = name

self.level = 1

self.__secret = 42

​

def reveal_secret(self):

return self.__secret

• Some programming languages support private attributes
• Private attributes cannot be accessed outside of the class
• Python doesn't support this directly
• "private" attributes start with double underscores
• Python name-mangles the attribute (changes the name)
• Can still be accessed

>>> ashs_pikachu.__secret

AttributeError

>>> ashs_pikachu._Pokemon__secret

42

>>> ashs_pikachu.reveal_secret()

42