Lab 8: OOP, Inheritance

Due by 11:59pm on Wednesday, March 15.

Starter Files

Download lab08.zip. Inside the archive, you will find starter files for the questions in this lab, along with a copy of the Ok autograder.

Topics

Consult this section if you need a refresher on the material for this lab. It's okay to skip directly to the questions and refer back here should you get stuck.


Inheritance

Python classes can implement a useful abstraction technique known as inheritance. To illustrate this concept, consider the following Dog and Cat classes. 
class Dog():
    def __init__(self, name, owner):
        self.is_alive = True
        self.name = name
        self.owner = owner
    def eat(self, thing):
        print(self.name + " ate a " + str(thing) + "!")
    def talk(self):
        print(self.name + " says woof!")

class Cat():
    def __init__(self, name, owner, lives=9):
        self.is_alive = True
        self.name = name
        self.owner = owner
        self.lives = lives
    def eat(self, thing):
        print(self.name + " ate a " + str(thing) + "!")
    def talk(self):
        print(self.name + " says meow!")

Notice that because dogs and cats share a lot of similar qualities, there is a lot of repeated code! To avoid redefining attributes and methods for similar classes, we can write a single base class from which the similar classes inherit. For example, we can write a class called Pet and redefine Dog as a subclass of Pet:

class Pet():
    def __init__(self, name, owner):
        self.is_alive = True    # It's alive!!!
        self.name = name
        self.owner = owner
    def eat(self, thing):
        print(self.name + " ate a " + str(thing) + "!")
    def talk(self):
        print(self.name)

class Dog(Pet):
    def talk(self):
        print(self.name + ' says woof!')

Inheritance represents a hierarchical relationship between two or more classes where one class is a (no relation to the Python is operator) more specific version of the other, e.g. a dog is a pet. Because Dog inherits from Pet, we didn't have to redefine __init__ or eat. However, since we want Dog to talk in a way that is unique to dogs, we did override the talk method.

We can use the super() function to refer to a class's superclass. For example, calling super() within the class definition of Dog allows us to access the same object but as if it were an instance of its superclass, in this case Pet. This is a little bit of a simplification, and if you're interested you can read more here.

Here's an example of an alternate equivalent definition of Dog that uses super() to explicitly call the __init__ method of the parent class:

class Dog(Pet):
    def __init__(self, name, owner):
        super().__init__(name, owner)
        # this is equivalent to calling Pet.__init__(self, name, owner)
    def talk(self):
        print(self.name + ' says woof!')

Keep in mind that creating the __init__ function shown above is actually not necessary, because creating a Dog instance will automatically call the __init__ method of Pet. Normally when defining an __init__ method in a subclass, we take some additional action to calling super().__init__. For example, we could add a new instance variable like the following:

def __init__(self, name, owner, has_floppy_ears):
    super().__init__(name, owner)
    self.has_floppy_ears = has_floppy_ears

Required Questions


Getting Started Videos

These videos may provide some helpful direction for tackling the coding problems on this assignment.

To see these videos, you should be logged into your berkeley.edu email.

YouTube link

What Would Python Display?

Q1: WWPD: Monster Truck

In the last lab's WWPD, we explored the behavior of the Car class. Here it is for reference:

Note: The Car class definition can also be found in car.py.

class Car:
    num_wheels = 4
    gas = 30
    headlights = 2
    size = 'Tiny'

    def __init__(self, make, model):
        self.make = make
        self.model = model
        self.color = 'No color yet. You need to paint me.'
        self.wheels = Car.num_wheels
        self.gas = Car.gas

    def paint(self, color):
        self.color = color
        return self.make + ' ' + self.model + ' is now ' + color

    def drive(self):
        if self.wheels < Car.num_wheels or self.gas <= 0:
            return 'Cannot drive!'
        self.gas -= 10
        return self.make + ' ' + self.model + ' goes vroom!'

    def pop_tire(self):
        if self.wheels > 0:
            self.wheels -= 1

    def fill_gas(self):
        self.gas += 20
        return 'Gas level: ' + str(self.gas)

For this lab, we will be referencing the MonsterTruck class below, a subclass of Car:

Note: The MonsterTruck class definition can also be found in car.py.

 class MonsterTruck(Car):
     size = 'Monster'

     def rev(self):
         print('Vroom! This Monster Truck is huge!')

     def drive(self):
         self.rev()
         return super().drive()

You can find the unlocking questions below.

Use Ok to test your knowledge with the following "What Would Python Display?" questions:

python3 ok -q wwpd-truck -u

Important: For all WWPD questions, type Function if you believe the answer is <function...>, Error if it errors, and Nothing if nothing is displayed.

>>> from car import *
>>> deneros_car = Car('Tesla', 'Model S')
>>> deneros_truck = MonsterTruck('Monster', 'Batmobile')
>>> deneros_car.size

______
'Tiny'

>>> deneros_car.drive()

______
'Tesla Model S goes vroom!'

>>> deneros_truck.size

______
'Monster'

>>> deneros_truck.drive()

______
Vroom! This Monster Truck is huge!
'Monster Batmobile goes vroom!'

>>> MonsterTruck.drive(deneros_truck)

______
Vroom! This Monster Truck is huge!
'Monster Batmobile goes vroom!'

>>> Car.drive(deneros_truck)

______
'Monster Batmobile goes vroom!'

>>> deneros_truck.gas

______
0

>>> MonsterTruck.rev(deneros_truck)

______
Vroom! This Monster Truck is huge!

>>> MonsterTruck.rev(deneros_car)

______
Vroom! This Monster Truck is huge!

>>> Car.rev(deneros_truck)

______
Error (Attibute Error)

Q2: WWPD: Inheritance ABCs

Use Ok to test your knowledge with the following "What Would Python Display?" questions:

python3 ok -q inheritance-abc -u

Important: For all WWPD questions, type Function if you believe the answer is <function...>, Error if it errors, and Nothing if nothing is displayed.

>>> class A:
...   x, y = 0, 0
...   def __init__(self):
...         return
>>> class B(A):
...   def __init__(self):
...         return
>>> class C(A):
...   def __init__(self):
...         return
>>> print(A.x, B.x, C.x)
______
>>> B.x = 2
>>> print(A.x, B.x, C.x)
______
>>> A.x += 1
>>> print(A.x, B.x, C.x)
______
>>> obj = C()
>>> obj.y = 1
>>> C.y == obj.y
______
>>> A.y = obj.y
>>> print(A.y, B.y, C.y, obj.y)
______

Coding Practice

Let's say we'd like to model a bank account that can handle interactions such as depositing funds or gaining interest on current funds. In the following questions, we will be building off of the Account class. Here's our current definition of the class:

class Account:
    """An account has a balance and a holder.
    >>> a = Account('John')
    >>> a.deposit(10)
    10
    >>> a.balance
    10
    >>> a.interest
    0.02
    >>> a.time_to_retire(10.25) # 10 -> 10.2 -> 10.404
    2
    >>> a.balance               # balance should not change
    10
    >>> a.time_to_retire(11)    # 10 -> 10.2 -> ... -> 11.040808032
    5
    >>> a.time_to_retire(100)
    117
    """
    max_withdrawal = 10
    interest = 0.02

    def __init__(self, account_holder):
        self.balance = 0
        self.holder = account_holder

    def deposit(self, amount):
        self.balance = self.balance + amount
        return self.balance

    def withdraw(self, amount):
        if amount > self.balance:
            return "Insufficient funds"
        if amount > self.max_withdrawal:
            return "Can't withdraw that amount"
        self.balance = self.balance - amount
        return self.balance

Q3: Retirement

Add a time_to_retire method to the Account class. This method takes in an amount and returns how many years the holder would need to wait in order for the current balance to grow to at least amount, assuming that the bank adds balance times the interest rate to the total balance at the end of every year.

    def time_to_retire(self, amount):
        """Return the number of years until balance would grow to amount."""
        assert self.balance > 0 and amount > 0 and self.interest > 0
        "*** YOUR CODE HERE ***"

Use Ok to test your code:

python3 ok -q Account

Q4: FreeChecking

Implement the FreeChecking class, which is like the Account class from lecture except that it charges a withdraw fee after 2 withdrawals. If a withdrawal is unsuccessful, it still counts towards the number of free withdrawals remaining, but no fee for the withdrawal will be charged.

Hint: Don't forget that FreeChecking inherits from Account! Check the Inheritance section in Topics for a refresher.

class FreeChecking(Account):
    """A bank account that charges for withdrawals, but the first two are free!
    >>> ch = FreeChecking('Jack')
    >>> ch.balance = 20
    >>> ch.withdraw(100)  # First one's free. Still counts as a free withdrawal even though it was unsuccessful
    'Insufficient funds'
    >>> ch.withdraw(3)    # Second withdrawal is also free
    17
    >>> ch.balance
    17
    >>> ch.withdraw(3)    # Ok, two free withdrawals is enough
    13
    >>> ch.withdraw(3)
    9
    >>> ch2 = FreeChecking('John')
    >>> ch2.balance = 10
    >>> ch2.withdraw(3) # No fee
    7
    >>> ch.withdraw(3)  # ch still charges a fee
    5
    >>> ch.withdraw(5)  # Not enough to cover fee + withdraw
    'Insufficient funds'
    """
    withdraw_fee = 1
    free_withdrawals = 2

    "*** YOUR CODE HERE ***"

Use Ok to test your code:

python3 ok -q FreeChecking

Magic: the Lambda-ing

In the next part of this lab, we will be implementing a card game! This game is inspired by the similarly named Magic: The Gathering.

Once you've implemented the game, you can start it by typing:

python3 cardgame.py

While playing the game, you can exit it and return to the command line with Ctrl-C or Ctrl-D.

This game uses several different files.

  • Code for all questions can be found in classes.py.
  • The game loop can be found in cardgame.py, and is responsible for running the game. You won't need to open or read this file to receive full credit.
  • If you want to modify your game later to add your own custom cards and decks, you can look in cards.py to see all the standard cards and the default deck; here, you can add more cards and change what decks you and your opponent use. If you're familiar with the original game, you may notice the cards were not created with balance in mind, so feel free to modify the stats and add or remove cards as desired.

Rules of the Game

Here's how the game goes:

There are two players. Each player has a hand of cards and a deck, and at the start of each round, each player draws a random card from their deck. If a player's deck is empty when they try to draw, they will automatically lose the game.

Cards have a name, an attack value, and a defense value. Each round, each player chooses one card to play from their own hands. The cards' power values are then calculated and compared. The card with the higher power wins the round. Each played card's power value is calculated as follows:

(player card's attack) - (opponent card's defense)

For example, let's say Player 1 plays a card with 2000 attack and 1000 defense and Player 2 plays a card with 1500 attack and 3000 defense. Their cards' powers are calculated as:

P1: 2000 - 3000 = 2000 - 3000 = -1000
P2: 1500 - 1000 = 1500 - 1000 = 500

So Player 2 would win this round.

The first player to win 8 rounds wins the match!

However, there are a few effects we can add (in the optional questions section) to make this game a more interesting. A card can be of type AI, Tutor, TA, or Instructor, and each type has a different effect when they are played. Note that when a card is played, the card is removed from the player's hand. This means that the card is no longer in the hand when the effect takes place. All effects are applied before power is calculated during that round:

  • An AICard will allow you to add the top two cards of your deck to your hand via drawing.
  • A TutorCard will add a copy of the first card in your hand to your hand, at the cost of automatically losing the current round.
  • A TACard discards the card with the highest power in your hand, and adds the discarded card's attack and defense to the played TACard's stats.
  • An InstructorCard can survive multiple rounds, as long as it has a non-negative attack or defense. However, at the beginning of each round that it is played, its attack and defense are reduced by 1000 each.

Feel free to refer back to these series of rules later on, and let's start making the game!

Q5: Making Cards

To play a card game, we're going to need to have cards, so let's make some! We're gonna implement the basics of the Card class first.

First, implement the Card class' constructor in classes.py. This constructor takes three arguments:

  • a string as the name of the card
  • an integer as the attack value of the card
  • an integer as the defense value of the card

Each Card instance should keep track of these values using instance attributes called name, attack, and defense.

You should also implement the power method in Card, which takes in another card as an input and calculates the current card's power. Refer to the Rules of the Game if you'd like a refresher on how power is calculated.

class Card:
    cardtype = 'Staff'

    def __init__(self, name, attack, defense):
        """
        Create a Card object with a name, attack,
        and defense.
        >>> staff_member = Card('staff', 400, 300)
        >>> staff_member.name
        'staff'
        >>> staff_member.attack
        400
        >>> staff_member.defense
        300
        >>> other_staff = Card('other', 300, 500)
        >>> other_staff.attack
        300
        >>> other_staff.defense
        500
        """
        "*** YOUR CODE HERE ***"

    def power(self, opponent_card):
        """
        Calculate power as:
        (player card's attack) - (opponent card's defense)
        >>> staff_member = Card('staff', 400, 300)
        >>> other_staff = Card('other', 300, 500)
        >>> staff_member.power(other_staff)
        -100
        >>> other_staff.power(staff_member)
        0
        >>> third_card = Card('third', 200, 400)
        >>> staff_member.power(third_card)
        0
        >>> third_card.power(staff_member)
        -100
        """
        "*** YOUR CODE HERE ***"

Use Ok to test your code:

python3 ok -q Card.__init__
python3 ok -q Card.power

Q6: Making a Player

Now that we have cards, we can make a deck, but we still need players to actually use them. We'll now fill in the implementation of the Player class.

A Player instance has three instance attributes:

  • name is the player's name. When you play the game, you can enter your name, which will be converted into a string to be passed to the constructor.
  • deck is an instance of the Deck class. You can draw from it using its .draw() method.
  • hand is a list of Card instances. Each player should start with 5 cards in their hand, drawn from their deck. Each card in the hand can be selected by its index in the list during the game. When a player draws a new card from the deck, it is added to the end of this list.

Complete the implementation of the constructor for Player so that self.hand is set to a list of 5 cards drawn from the player's deck.

Next, implement the draw and play methods in the Player class. The draw method draws a card from the deck and adds it to the player's hand. The play method removes and returns a card from the player's hand at the given index.

Hint: use methods from the Deck class wherever possible when attempting to draw from the deck when implementing Player.__init__ and Player.draw.

class Player:
    def __init__(self, deck, name):
        """Initialize a Player object.
        A Player starts the game by drawing 5 cards from their deck. Each turn,
        a Player draws another card from the deck and chooses one to play.
        >>> test_card = Card('test', 100, 100)
        >>> test_deck = Deck([test_card.copy() for _ in range(6)])
        >>> test_player = Player(test_deck, 'tester')
        >>> len(test_deck.cards)
        1
        >>> len(test_player.hand)
        5
        """
        self.deck = deck
        self.name = name
        "*** YOUR CODE HERE ***"

    def draw(self):
        """Draw a card from the player's deck and add it to their hand.
        >>> test_card = Card('test', 100, 100)
        >>> test_deck = Deck([test_card.copy() for _ in range(6)])
        >>> test_player = Player(test_deck, 'tester')
        >>> test_player.draw()
        >>> len(test_deck.cards)
        0
        >>> len(test_player.hand)
        6
        """
        assert not self.deck.is_empty(), 'Deck is empty!'
        "*** YOUR CODE HERE ***"

    def play(self, index):
        """Remove and return a card from the player's hand at the given INDEX.
        >>> from cards import *
        >>> test_player = Player(standard_deck, 'tester')
        >>> ta1, ta2 = TACard("ta_1", 300, 400), TACard("ta_2", 500, 600)
        >>> tutor1, tutor2 = TutorCard("t1", 200, 500), TutorCard("t2", 600, 400)
        >>> test_player.hand = [ta1, ta2, tutor1, tutor2]
        >>> test_player.play(0) is ta1
        True
        >>> test_player.play(2) is tutor2
        True
        >>> len(test_player.hand)
        2
        """
        "*** YOUR CODE HERE ***"

Use Ok to test your code:

python3 ok -q Player.__init__
python3 ok -q Player.draw
python3 ok -q Player.play

After you complete this problem, you'll be able to play a working version of the game! Type:

python3 cardgame.py

to start a game of Magic: The Lambda-ing!

This version doesn't have the effects for different cards yet. To get those working, you can implement the optional questions below.

Check Your Score Locally

You can locally check your score on each question of this assignment by running

python3 ok --score

This does NOT submit the assignment! When you are satisfied with your score, submit the assignment to Gradescope to receive credit for it.

Submit

Make sure to submit this assignment by uploading any files you've edited to the appropriate Gradescope assignment. For a refresher on how to do this, refer to Lab 00.

For this lab, you will submit two files, classes.py and lab08.py. If you do not submit both files, you will not pass our autograder tests! For Mac users, you can select multiple files by holding down the command button and then clicking on the files you want to select. For Windows users, you can select multiple files by holding down the ctrl button and then clicking on the files you want to select. See the Ed post for more details.

Optional Questions

To make the card game more interesting, let's add effects to our cards! We can do this by implementing an effect function for each card class, which takes in the opponent card, the current player, and the opponent player. Remember that by the time effect is called, the played card is no longer in the player's hand.

You can find the following questions in classes.py.

Important: For the following sections, do not overwrite any lines already provided in the code.

Q7: AIs: Resourceful Resources

In the AICard class, implement the effect method for AIs. An AICard will allow you to add the top two cards of your deck to your hand via drawing from your deck.

Once you have finished writing your code for this problem, set implemented to True so that the text is printed when playing an AICard! This is specifically for the AICard! For future questions, make sure to look at the problem description carefully to know when to reassign any pre-designated variables.

class AICard(Card):
    cardtype = 'AI'

    def effect(self, opponent_card, player, opponent):
        """
        Add the top two cards of your deck to your hand via drawing.
        Once you have finished writing your code for this problem,
        set implemented to True so that the text is printed when
        playing an AICard.

        >>> from cards import *
        >>> player1, player2 = Player(standard_deck.copy(), 'p1'), Player(standard_deck.copy(), 'p2')
        >>> opponent_card = Card("other", 500, 500)
        >>> test_card = AICard("AI Card", 500, 500)
        >>> initial_deck_length = len(player1.deck.cards)
        >>> initial_hand_size = len(player1.hand)
        >>> test_card.effect(opponent_card, player1, player2)
        AI Card allows me to draw two cards!
        >>> initial_hand_size == len(player1.hand) - 2
        True
        >>> initial_deck_length == len(player1.deck.cards) + 2
        True
        """
        "*** YOUR CODE HERE ***"
        implemented = False
        # You should add your implementation above this.
        if implemented:
            print(f"{self.name} allows me to draw two cards!")

Use Ok to test your code:

python3 ok -q AICard.effect

In the TutorCard class, implement the effect method for Tutors. A TutorCard will add a copy of the first card in your hand to your hand, at the cost of automatically losing the current round. Note that if there are no cards in hand, a TutorCard will not add any cards to the hand, but must still lose the round.

To implement the "losing" functionality, it is sufficient to override Card's power method to return -float('inf') in the TutorCard class. In addition, be sure to add copies of cards, instead of the chosen card itself! Class methods may come in handy.

class TutorCard(Card):
    cardtype = 'Tutor'

    def effect(self, opponent_card, player, opponent):
        """
        Add a copy of the first card in your hand
        to your hand, at the cost of losing the current
        round. If there are no cards in hand, this card does
        not add any cards, but still loses the round.  To
        implement the second part of this effect, a Tutor
        card's power should be less than all non-Tutor cards.

        >>> from cards import *
        >>> player1, player2 = Player(standard_deck.copy(), 'p1'), Player(standard_deck.copy(), 'p2')
        >>> opponent_card = Card("other", 500, 500)
        >>> test_card = TutorCard("Tutor Card", 10000, 10000)
        >>> player1.hand = [Card("card1", 0, 100), Card("card2", 100, 0)]
        >>> test_card.effect(opponent_card, player1, player2)
        Tutor Card allows me to add a copy of a card to my hand!
        >>> print(player1.hand)
        [card1: Staff, [0, 100], card2: Staff, [100, 0], card1: Staff, [0, 100]]
        >>> player1.hand[0] is player1.hand[2] # must add a copy!
        False
        >>> player1.hand = []
        >>> test_card.effect(opponent_card, player1, player2)
        >>> print(player1.hand) # must not add a card if not available
        []
        >>> test_card.power(opponent_card) < opponent_card.power(test_card)
        True
        """
        "*** YOUR CODE HERE ***"
        added = False
        # You should add your implementation above this.
        if added:
            print(f"{self.name} allows me to add a copy of a card to my hand!")

    "*** YOUR CODE HERE ***"

Use Ok to test your code:

python3 ok -q TutorCard.effect

Q9: TAs: Power Transfer

In the TACard class, implement the effect method for TAs. A TACard discards the card with the highest power in your hand, and adds the discarded card's attack and defense to the played TACard's stats. Discarding a card removes the card from your hand. If there are no cards in hand, the TACard should not do anything for its effect.

class TACard(Card):
    cardtype = 'TA'

    def effect(self, opponent_card, player, opponent, arg=None):
        """
        Discard the card with the highest `power` in your hand,
        and add the discarded card's attack and defense
        to this card's own respective stats.

        >>> from cards import *
        >>> player1, player2 = Player(standard_deck.copy(), 'p1'), Player(standard_deck.copy(), 'p2')
        >>> opponent_card = Card("other", 500, 500)
        >>> test_card = TACard("TA Card", 500, 500)
        >>> player1.hand = []
        >>> test_card.effect(opponent_card, player1, player2) # if no cards in hand, no effect.
        >>> print(test_card.attack, test_card.defense)
        500 500
        >>> player1.hand = [Card("card1", 0, 100), TutorCard("tutor", 10000, 10000), Card("card3", 100, 0)]
        >>> test_card.effect(opponent_card, player1, player2) # must use card's power method.
        TA Card discards card3 from my hand to increase its own power!
        >>> print(player1.hand)
        [card1: Staff, [0, 100], tutor: Tutor, [10000, 10000]]
        >>> print(test_card.attack, test_card.defense)
        600 500
        """
        "*** YOUR CODE HERE ***"
        best_card = None
        # You should add your implementation above this.
        if best_card:
            print(f"{self.name} discards {best_card.name} from my hand to increase its own power!")

Use Ok to test your code:

python3 ok -q TACard.effect

Q10: Instructors: Immovable

In the InstructorCard class, implement the effect method for Instructors. An InstructorCard can survive multiple rounds, as long as it has a non-negative attack or defense at the end of a round. However, at the beginning of each round that it is played (including the first time!), its attack and defense are permanently reduced by 1000 each.

To implement the "survive" functionality, the InstructorCard should re-add itself to the player's hand.

class InstructorCard(Card):
    cardtype = 'Instructor'

    def effect(self, opponent_card, player, opponent, arg=None):
        """
        Survives multiple rounds, as long as it has a non-negative
        attack or defense at the end of a round. At the beginning of the round,
        its attack and defense are permanently reduced by 1000 each.
        If this card would survive, it is added back to the hand.

        >>> from cards import *
        >>> player1, player2 = Player(standard_deck.copy(), 'p1'), Player(standard_deck.copy(), 'p2')
        >>> opponent_card = Card("other", 500, 500)
        >>> test_card = InstructorCard("Instructor Card", 1000, 1000)
        >>> player1.hand = [Card("card1", 0, 100)]
        >>> test_card.effect(opponent_card, player1, player2)
        Instructor Card returns to my hand!
        >>> print(player1.hand) # survives with non-negative attack
        [card1: Staff, [0, 100], Instructor Card: Instructor, [0, 0]]
        >>> player1.hand = [Card("card1", 0, 100)]
        >>> test_card.effect(opponent_card, player1, player2)
        >>> print(player1.hand)
        [card1: Staff, [0, 100]]
        >>> print(test_card.attack, test_card.defense)
        -1000 -1000
        """
        "*** YOUR CODE HERE ***"
        re_add = False
        # You should add your implementation above this.
        if re_add:
            print(f"{self.name} returns to my hand!")

Use Ok to test your code:

python3 ok -q InstructorCard.effect

After you complete this problem, you'll have a fully functional game of Magic: The Lambda-ing! This doesn't have to be the end, though; we encourage you to get creative with more card types, effects, and even adding more custom cards to your deck!