Lab 6: Nonlocal & Object-Oriented Programming

Due by 11:59pm on Monday, October 14.

Starter Files

Download lab06.zip. Inside the archive, you will find starter files for the questions in this lab, along with a copy of the Ok autograder.

Submission

By the end of this lab, you should have submitted the lab with python3 ok --submit. You may submit more than once before the deadline; only the final submission will be graded. Check that you have successfully submitted your code on okpy.org.

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.

Nonlocal

We say that a variable defined in a frame is local to that frame. A variable is nonlocal to a frame if it is defined in the environment that the frame belongs to but not the frame itself, i.e. in its parent or ancestor frame.

So far, we know that we can access variables in parent frames:

def make_adder(x):
    """ Returns a one-argument function that returns the result of
    adding x and its argument. """
    def adder(y):
        return x + y
    return adder

Here, when we call make_adder, we create a function adder that is able to look up the name x in make_adder's frame and use its value.

However, we haven't been able to modify variable in parent frames. Consider the following function:

def make_withdraw(balance):
    """Returns a function which can withdraw
    some amount from balance

    >>> withdraw = make_withdraw(50)
    >>> withdraw(25)
    25
    >>> withdraw(25)
    0
    """
    def withdraw(amount):
        if amount > balance:
            return "Insufficient funds"
        balance = balance - amount
        return balance
    return withdraw

The inner function withdraw attempts to update the variable balance in its parent frame. Running this function's doctests, we find that it causes the following error:

UnboundLocalError: local variable 'balance' referenced before assignment

Why does this happen? When we execute an assignment statement, remember that we are either creating a new binding in our current frame or we are updating an old one in the current frame. For example, the line balance = ... in withdraw, is creating the local variable balance inside withdraw's frame. This assignment statement tells Python to expect a variable called balance inside withdraw's frame, so Python will not look in parent frames for this variable. However, notice that we tried to compute balance - amount before the local variable was created! That's why we get the UnboundLocalError.

To avoid this problem, we introduce the nonlocal keyword. It allows us to update a variable in a parent frame!

Some important things to keep in mind when using nonlocal

  • nonlocal cannot be used with global variables (names defined in the global frame).
  • If no nonlocal variable is found with the given name, a SyntaxError is raised.
  • A name that is already local to a frame cannot be declared as nonlocal.

Consider this improved example:

def make_withdraw(balance):
    """Returns a function which can withdraw
    some amount from balance

    >>> withdraw = make_withdraw(50)
    >>> withdraw(25)
    25
    >>> withdraw(25)
    0
    """
    def withdraw(amount):
        nonlocal balance
        if amount > balance:
            return "Insufficient funds"
        balance = balance - amount
        return balance
    return withdraw

The line nonlocal balance tells Python that balance will not be local to this frame, so it will look for it in parent frames. Now we can update balance without running into problems.


Object-Oriented Programming

Object-oriented programming (OOP) is a style of programming that allows you to think of code in terms of "objects." Here's an example of a Car class:

class Car(object):
    num_wheels = 4

    def __init__(self, color):
        self.wheels = Car.num_wheels
        self.color = color

    def drive(self):
        if self.wheels <= Car.num_wheels:
            return self.color + ' car cannot drive!'
        return self.color + ' car goes vroom!'

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

Here's some terminology:

  • class: a blueprint for how to build a certain type of object. The Car class (shown above) describes the behavior and data that all Car objects have.

  • instance: a particular occurrence of a class. In Python, we create instances of a class like this:

    >>> my_car = Car('red')

    my_car is an instance of the Car class.

  • attribute or field: a variable that belongs to the class. Think of an attribute as a quality of the object: cars have wheels and color, so we have given our Car class self.wheels and self.color attributes. We can access attributes using dot notation:

    >>> my_car.color
'red'
>>> my_car.wheels
4

  • method: Methods are just like normal functions, except that they are tied to an instance or a class. Think of a method as a "verb" of the class: cars can drive and also pop their tires, so we have given our Car class the methods drive and pop_tire. We call methods using dot notation:

    >>> my_car = Car('red')
>>> my_car.drive()
'red car goes vroom!'

  • constructor: As with data abstraction, constructors describe how to build an instance of the class. Most classes have a constructor. In Python, the constructor of the class defined as __init__. For example, here is the Car class's constructor:

    def __init__(self, color):
    self.wheels = Car.num_wheels
    self.color = color

    The constructor takes in one argument, color. As you can see, the constructor also creates the self.wheels and self.color attributes.

  • self: in Python, self is the first parameter for many methods (in this class, we will only use methods whose first parameter is self). When a method is called, self is bound to an instance of the class. For example:

    >>> my_car = Car('red')
>>> car.drive()

    Notice that the drive method takes in self as an argument, but it looks like we didn't pass one in! This is because the dot notation implicitly passes in car as self for us.


Required Questions

Nonlocal Codewriting

For the following question, write your code in lab06.py.

Q1: Make Adder Increasing

Write a function which takes in an integer n and returns a one-argument function. This function should take in some value x and return n + x the first time it is called, similar to make_adder. The second time it is called, however, it should return n + x + 1, then n + x + 2 the third time, and so on.

def make_adder_inc(n):
    """
    >>> adder1 = make_adder_inc(5)
    >>> adder2 = make_adder_inc(6)
    >>> adder1(2) 
    7
    >>> adder1(2) # 5 + 2 + 1
    8
    >>> adder1(10) # 5 + 10 + 2
    17
    >>> [adder1(x) for x in [1, 2, 3]]
    [9, 11, 13]
    >>> adder2(5)
    11
    """

    "*** YOUR CODE HERE ***"

    def adder(x):
        nonlocal n
        value = n + x
        n = n + 1
        return value
    return adder

Use Ok to test your code:

python3 ok -q make_adder_inc

WWPD

Q2: Using the Car class

Here is the full definition of the Car class from the car example above in car.py:

class Car(object):
    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)

Use Ok to test your knowledge with the following What would Python Display questions.

python3 ok -q wwpd-car -u

If an error occurs, type Error. If nothing is displayed, type Nothing.

>>> deneros_car = Car('Tesla', 'Model S')
>>> deneros_car.model

______
'Model S'

>>> deneros_car.gas = 10
>>> deneros_car.drive()

______
'Tesla Model S goes vroom!'

>>> deneros_car.drive()

______
'Cannot drive!'

>>> deneros_car.fill_gas()

______
'Gas level: 20'

>>> deneros_car.gas

______
20

>>> Car.gas

______
30
>>> deneros_car = Car('Tesla', 'Model S')
>>> deneros_car.wheels = 2
>>> deneros_car.wheels

______
2

>>> Car.num_wheels

______
4

>>> deneros_car.drive()

______
'Cannot drive!'

>>> Car.drive()

______
Error (TypeError)

>>> Car.drive(deneros_car)

______
'Cannot drive!'

For the following, we reference the MonsterTruck class, also in car.py:

 class MonsterTruck(Car):
     size = 'Monster'

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

     def drive(self):
         self.rev()
         return Car.drive(self)
>>> deneros_car = MonsterTruck('Monster', 'Batmobile')
>>> deneros_car.drive()

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

>>> Car.drive(deneros_car)

______
'Monster Batmobile goes vroom!'

>>> MonsterTruck.drive(deneros_car)

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

>>> Car.rev(deneros_car)

______
Error (AttributeError)

Magic: The Lambda-ing

In the next part of this lab, we will be implementing a card game!

You can start the game by typing:

python3 cardgame.py

This game doesn't work yet. If we run this right now, the code will error, since we haven't implemented anything yet. When it's working, you can exit the game and return to the command line with Ctrl-C or Ctrl-D.

This game uses several different files.

  • Code for all the questions in this lab can be found in classes.py.
  • Some utility for the game can be found in cardgame.py, but you won't need to open or read this file. This file doesn't actually mutate any instances directly - instead, it calls methods of the different classes, maintaining a strict abstraction barrier.
  • 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. The cards were not created with balance in mind, so feel free to modify the stats and add/remove cards as desired.

Rules of the Game This game is a little involved, though not nearly as much as its namesake. Here's how it 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 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 stat, and a defense stat. Each round, each player chooses one card to play from their own hands. 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) / 2

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

P1: 2000 - 3000/2 = 2000 - 1500 = 500
P2: 1500 - 1000/2 = 1500 - 500 = 1000

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 bit more interesting. Cards are split into Tutor, TA, and Professor types, and each type has a different effect when they're played. All effects are applied before power is calculated during that round:

  • A Tutor will cause the opponent to discard and re-draw the first 3 cards in their hand.
  • A TA will swap the opponent card's attack and defense.
  • A Professor adds the opponent card's attack and defense to all cards in their deck and then remove all cards in the opponent's deck that share its attack or defense!

These are a lot of rules to remember, so refer back here if you need to review them, and let's start making the game!

Q3: 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:

  • the name of the card, a string
  • the attack stat of the card, an integer
  • the defense stat of the card, an integer

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. Check the Rules section if you want a refresher on how power is calculated.

class Card(object):
    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 ***"

        self.name = name
        self.attack = attack
        self.defense = defense

    def power(self, other_card):
        """
        Calculate power as:
        (player card's attack) - (opponent card's defense)/2
        where other_card is the opponent's card.
        >>> staff_member = Card('staff', 400, 300)
        >>> other_staff = Card('other', 300, 500)
        >>> staff_member.power(other_staff)
        150.0
        >>> other_staff.power(staff_member)
        150.0
        >>> third_card = Card('third', 200, 400)
        >>> staff_member.power(third_card)
        200.0
        >>> third_card.power(staff_member)
        50.0
        """

        "*** YOUR CODE HERE ***"

        return self.attack - other_card.defense / 2

Use Ok to test your code:

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

Q4: 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.

Call deck.draw() when implementing Player.__init__ and Player.draw. Don't worry about how this function works - leave it all to the abstraction!

class Player(object):
    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 ***"

        self.hand = [deck.draw() for _ in range(5)]

    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 ***"

        self.hand.append(self.deck.draw())

    def play(self, card_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 ***"

        return self.hand.pop(card_index)

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, try out the optional questions below.

Optional Questions

Note: These questions use inheritance, which has not been covered in lecture yet. It will be covered on Wednesday, 10/9.

For the following sections, do not overwrite any lines already provided in the code. Additionally, make sure to uncomment any calls to print once you have implemented each method. These are used to display information to the user, and changing them may cause you to fail tests that you would otherwise pass.

Q5: Tutors: Flummox

To really make this card game interesting, our cards should have effects! We'll do this with the effect function for cards, which takes in the opponent card, the current player, and the opponent player.

Implement the effect method for Tutors, which causes the opponent to discard the first 3 cards in their hand and then draw 3 new cards. Assume there at least 3 cards in the opponent's hand and at least 3 cards in the opponent's deck.

Remember to uncomment the call to print once you're done!

class TutorCard(Card):
    cardtype = 'Tutor'

    def effect(self, other_card, player, opponent):
        """
        Discard the first 3 cards in the opponent's hand and have
        them draw the same number of cards from their deck.
        >>> from cards import *
        >>> player1, player2 = Player(player_deck, 'p1'), Player(opponent_deck, 'p2')
        >>> other_card = Card('other', 500, 500)
        >>> tutor_test = TutorCard('Tutor', 500, 500)
        >>> initial_deck_length = len(player2.deck.cards)
        >>> tutor_test.effect(other_card, player1, player2)
        p2 discarded and re-drew 3 cards!
        >>> len(player2.hand)
        5
        >>> len(player2.deck.cards) == initial_deck_length - 3
        True
        """

        "*** YOUR CODE HERE ***"

        opponent.hand = opponent.hand[3:]
        for _ in range(3):
            opponent.draw()
        #Uncomment the line below when you've finished implementing this method!
        #print('{} discarded and re-drew 3 cards!'.format(opponent.name))

Use Ok to test your code:

python3 ok -q TutorCard.effect

Q6: TAs: Shift

Let's add an effect for TAs now! Implement the effect method for TAs, which swaps the attack and defense of the opponent's card.

class TACard(Card):
    cardtype = 'TA'

    def effect(self, other_card, player, opponent):
        """
        Swap the attack and defense of an opponent's card.
        >>> from cards import *
        >>> player1, player2 = Player(player_deck, 'p1'), Player(opponent_deck, 'p2')
        >>> other_card = Card('other', 300, 600)
        >>> ta_test = TACard('TA', 500, 500)
        >>> ta_test.effect(other_card, player1, player2)
        >>> other_card.attack
        600
        >>> other_card.defense
        300
        """

        "*** YOUR CODE HERE ***"

        other_card.attack, other_card.defense = other_card.defense, other_card.attack

Use Ok to test your code:

python3 ok -q TACard.effect

Q7: The Professor Arrives

A new challenger has appeared! Implement the effect method for the Professor, who adds the opponent card's attack and defense to all cards in the player's deck and then removes all cards in the opponent's deck that have the same attack or defense as the opponent's card.

Note: You might run into trouble when you mutate a list as you're iterating through it. Try iterating through a copy instead! You can use slicing to copy a list:

  >>> lst = [1, 2, 3, 4]
  >>> copy = lst[:]
  >>> copy
  [1, 2, 3, 4]
  >>> copy is lst
  False
class ProfessorCard(Card):
    cardtype = 'Professor'

    def effect(self, other_card, player, opponent):
        """
        Adds the attack and defense of the opponent's card to
        all cards in the player's deck, then removes all cards
        in the opponent's deck that share an attack or defense
        stat with the opponent's card.
        >>> test_card = Card('card', 300, 300)
        >>> professor_test = ProfessorCard('Professor', 500, 500)
        >>> opponent_card = test_card.copy()
        >>> test_deck = Deck([test_card.copy() for _ in range(8)])
        >>> player1, player2 = Player(test_deck.copy(), 'p1'), Player(test_deck.copy(), 'p2')
        >>> professor_test.effect(opponent_card, player1, player2)
        3 cards were discarded from p2's deck!
        >>> [(card.attack, card.defense) for card in player1.deck.cards]
        [(600, 600), (600, 600), (600, 600)]
        >>> len(player2.deck.cards)
        0
        """
        orig_opponent_deck_length = len(opponent.deck.cards)

        "*** YOUR CODE HERE ***"

        for card in player.deck.cards:
            card.attack += other_card.attack
            card.defense += other_card.defense
        for card in opponent.deck.cards[:]:
            if card.attack == other_card.attack or card.defense == other_card.defense:
                opponent.deck.cards.remove(card)
        discarded = orig_opponent_deck_length - len(opponent.deck.cards)
        if discarded:
            #Uncomment the line below when you've finished implementing this method!
            #print('{} cards were discarded from {}\'s deck!'.format(discarded, opponent.name))
            return

Use Ok to test your code:

python3 ok -q ProfessorCard.effect

After you complete this problem, we'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!

Q8: Nonlocal Environment Diagram

Draw the environment diagram that results from running the following code.

def moon(f):
    sun = 0
    moon = [sun]
    def run(x):
        nonlocal sun, moon
        def sun(sun):
            return [sun]
        y = f(x)
        moon.append(sun(y))
        return moon[0] and moon[1]
    return run

moon(lambda x: moon)(1)

After you've done it on your own, generate an environment diagram in python tutor to check your answer.