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 allCar
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 theCar
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
classself.wheels
andself.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 methodsdrive
andpop_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 theCar
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 theself.wheels
andself.color
attributes.self
: in Python,self
is the first parameter for many methods (in this class, we will only use methods whose first parameter isself
). 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 inself
as an argument, but it looks like we didn't pass one in! This is because the dot notation implicitly passes incar
asself
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 thePlayer
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 theDeck
class. You can draw from it using its.draw()
method.hand
is a list ofCard
instances. Each player should start with 5 cards in their hand, drawn from theirdeck
. 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 implementingPlayer.__init__
andPlayer.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.