Starter Files

Download lab4.zip. Inside the archive, you will find starter files for the questions in this lab.

Submission

By the end of this lab, submit the lab4.py and classes.py files to Poseidon. For checkoff, post your answers to question 9 and 10 as a private post visible to all instructors on piazza.

To receive credit for this lab, you must complete Questions 1, 3 and 4 and submit.
To be checked off, make a private post to Instructors on Piazza. The questions are at the end of this page.
The remaining questions are optional. It is recommended that you complete these problems in your own time.

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
    """
    def withdraw():
        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
    """
    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

In this lab we'll be diving into object-oriented programming (OOP), a model of programming that allows you to think of data in terms of "objects" with their own characteristics and actions, just like objects in real life! This is very powerful and allows you to create objects that are specific to your program - you can read up on all the details here.

OOP Example: `Car` Class

Instructor Eric is running late, and needs to get from Gweru to Bulawayo before lecture starts. He'd take ZUPCO, but that will take too long. It'd be great if he had a car. A monster truck would be best, but a car will do -- for now...

In car.py, you'll find a class called Car. A class is a blueprint for creating objects of that type. In this case, the Car class statement tells us how to create Car objects.

Constructor

Let's build Instructor Eric a car! Don't worry, you won't need to do any physical work -- the constructor will do it for you. The constructor of a class is a function that creates an instance, or a single occurrence, of the object outlined by the class. In Python, the constructor method is named __init__. Note that there must be two underscores on each side of init. The Car class' constructor looks like this:

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

The __init__ method for Car has three parameters. The first one, self, is automatically bound to the newly created Car object. The second and third parameters, make and model, are bound to the arguments passed to the constructor, meaning when we make a Car object, we must provide two arguments. Don't worry about the code inside the body of the constructor for now.

Let's make our car. Eric would like to drive a Tesla Model S to lecture. We can construct an instance of Car with 'Tesla' as the make and 'Model S' as the model. Rather than calling __init__ explicitly, Python allows us to make an instance of a class by using the name of the class.

>>> erics_car = Car('Tesla', 'Model S')

Here, 'Tesla' is passed in as the make, and 'Model S' as the model. Note that we don't pass in an argument for self, since its value is always the object being created. An object is an instance of a class. In this case, erics_car is now bound to a Car object or, in other words, an instance of the Car class.

Attributes and Dot Notation

So how are the make and model of Eric's car actually stored? Let's talk about attributes of instances and classes. Here's a snippet of the code in car.py with the instance and class attributes in the Car class:

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)

In the first two lines of the constructor, the name self.make is bound to the first argument passed to the constructor and self.model is bound to the second. These are two examples of instance attributes. An instance attribute is a quality or variable that is specific to an instance, and not the class itself! Instance attributes are accessed using dot notation (separating the instance and attribute with a period) with an instance. In this case, self is bound to our instance, so self.model references our instance's model.

Our car has other instance attributes too, like color and wheels. As instance attributes, the make, model, and color of erics_car do not affect the make, model, and color of other cars.

On the other hand, a class attribute is a quality that is shared among all instances of the class. For example, the Car class has four class attributes defined at the beginning of a class: num_wheels = 4, gas = 30, headlights = 2 and size = 'Tiny'. The first says that all cars have 4 wheels.

You might notice in the __init__ method of the Car class, the instance attribute gas is initialized to the value of Car.gas, the class attribute. Why don't we just use the class attribute, then? The reason is because each Car's gas attribute needs to be able to change independently of each other.

Class attributes can also be accessed using dot notation, both on an instance and on the class name itself. For example, we can access the class attribute size of Car like this:

>>> Car.size
'Tiny'

And in the following line, we access erics_car's color attribute:

>>> erics_car.color
'No color yet. You need to paint me.'

Methods

Let's use the paint method from the Car class. Methods are functions that are specific to a class; only an instance of the class can use them. We've already seen one method: __init__! Think of methods as actions or abilities of objects. How do we call methods on an instance? You guessed it, dot notation!

>>> erics_car.paint('black')
'Tesla Model S is now black'
>>> erics_car.color
'black'

You can also call methods using the class name and dot notation; for example,

>>> Car.paint(erics_car, 'red')
'Tesla Model S is now red'

Inheritance

Instructor Eric's red Tesla is pretty cool, but he wants a bigger car! How about we create a monster truck for him instead? In car.py, we've defined a MonsterTruck class. Let's look at the code for MonsterTruck:

class MonsterTruck(Car):
    size = 'Monster'

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

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

The class MonsterTruck is defined as class MonsterTruck(Car):, meaning its superclass is Car. Likewise, the class MonsterTruck is a subclass of the Car class. That means the MonsterTruck class inherits all the attributes and methods that were defined in Car, including its constructor!

Inheritance makes setting up a hierarchy of classes easier because the amount of code you need to write to define a new class of objects is reduced. You only need to add (or override) new attributes or methods that you want to be unique from those in the superclass.

>>> erics_car.size
'Tiny'
>>> erics_truck.size
'Monster'

Required Questions

Nonlocal Codewriting

For the following question, write your code in lab4.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

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)

Test your knowledge with the following What would Python Display questions.

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

>>> erics_car = Car('Tesla', 'Model S')
>>> erics_car.model
______
'Model S'
>>> erics_car.gas = 10
>>> erics_car.drive()
______
'Tesla Model S goes vroom!'
>>> erics_car.drive()
______
'Cannot drive!'
>>> erics_car.fill_gas()
______
'Gas level: 20'
>>> erics_car.gas
______
20
>>> Car.gas
______
30

>>> erics_car.wheels = 2
>>> erics_car.wheels
______
2
>>> Car.num_wheels
______
4
>>> erics_car.drive()
______
'Cannot drive!'
>>> Car.drive()
______
Error (TypeError)
>>> Car.drive(erics_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)

>>> erics_car = MonsterTruck('Monster', 'Batmobile')
>>> erics_car.drive()
______
Vroom! This Monster Truck is huge!
'Monster Batmobile goes vroom!'
>>> Car.drive(erics_car)
______
'Monster Batmobile goes vroom!'
>>> MonsterTruck.drive(erics_car)
______
Vroom! This Monster Truck is huge!
'Monster Batmobile goes vroom!'
>>> Car.rev(erics_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

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.
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.

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)

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!

Optional Questions

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.

Q5: Tutors: Flummox

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 are at least 3 cards in the opponent's hand and at least 3 cards in the opponent's deck.

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))

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

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        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

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.

Checkoff

Q9: Mutating Lists

First, let's say we have a function whose sole goal is to set the element at index origin to 0 and to set the element at dest to 1. Is there anything wrong with the following code, and if so, what?

>>> def mutate_lst(lst, origin, dest):
...     # First let's set the original location to 0
...     counter = 0
...     for i in lst:
...         if counter == origin:
...             i = 0
...         counter += 1
...     # Now let's set the element at dest to 1
...     counter = 0
...     for i in lst:
...         if counter == dest:
...             i = 1
...         counter += 1
...     return None
...

A for-loop like this is equivalent to doing this:

>>> counter = 0
>>> while counter < len(lst):
...     i = lst[counter]
...     if counter == origin:
...         i = 0
...     counter += 1
...

However, what we have done is change the value of i, but not the actual element in the list itself. To fix this we could do this instead

>>> counter = 0
>>> while counter < len(lst):
...     if counter == origin:
...         lst[counter] = 0
...     counter += 1
...

Q10: Mutating Trees

Say we want to implement mutate_tree which is a function that squares every value in a tree and updates the labels of each node. Is there anything wrong with the following implementation? If so, what?

>>> def mutate_tree(t):
...     label(t) = label(t) * label(t)
...     if is_leaf(t):
...         return None
...     else:
...         for b in branches(t):
...             mutate_tree(b)
...

label(t) is a selector, which returns information about the given abstract data type. If you tried to use this function you would get the following error: SyntaxError: can't assign to function call.

Walk through Python's order of evaluation pedantically: evaluate the right side first. label(lst) evaluates to 1, so we get 1 * 1 on the right side. Now bind 1 to the left side. But what is the left side? label(lst) is 1 and you cannot assign a value to an integer like this (actually Python never even evaluates label(lst) on either side of the assignment--it simply detects an assignment to a function call and automatically errors). So how do you mutate labels using our implementation of our current implementation of trees? You cannot without violating abstraction barriers. This is why we have asked you to create new trees instead of modifying the original trees on questions that involve changing labels.