Lab 6: Object-Oriented Programming

Due at 11:59pm on Friday, 03/02/2018.

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.

  • Questions 1, 2, and 3, must be completed in order to receive credit for this lab. Questions 2 is done in the data.py file and Question 3 is done in classes.py.
  • Question 4 is optional. It is recommended that you complete this problem if you finish the required portion early. The starter code for this problem is in lab06_extra.py.
  • Questions 5 through 8 are optional. It is recommended that you complete these problems on your own time. The starter code for these problems is in classes.py.

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_counter():
    """Makes a counter function.

    >>> counter = make_counter()
    >>> counter()
    1
    >>> counter()
    2
    """
    count = 0
    def counter():
        count = count + 1
        return count
    return counter

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

UnboundLocalError: local variable 'count' 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 count = ... in counter, is creating the local variable count inside counter's frame. This assignment statement tells Python to expect a variable called count inside counter's frame, so Python will not look in parent frames for this variable. However, notice that we tried to compute count + 1 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_counter():
    """Makes a counter function.

    >>> counter = make_counter()
    >>> counter()
    1
    >>> counter()
    2
    """
    count = 0
    def counter():
        nonlocal count
        count = count + 1
        return count
    return counter

The line nonlocal count tells Python that count will not be local to this frame, so it will look for it in parent frames. Now we can update count 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

Professor Hilfinger is running late, and needs to get from San Francisco to Berkeley before lecture starts. He'd take BART, 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 Professor Hilfinger 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 one 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. Professor Hilfinger 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.

>>> hilfingers_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, hilfingers_car is now bound to a Car object or, in other words, an instance of the Car class.

Attributes

So how are the make and model of Professor Hilfinger's car actually stored? Let's talk about attributes of instances and classes. Here's a snippet of the code in car.py of 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

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 that is specific to an instance, and thus can only be accessed using dot notation (separating the instance and attribute with a period) on 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 hilfingers_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. If one Car drives for a while, it should use up some gas, and that Car instance should reflect that by having a lower gas value. However, all other Cars shouldn't lose any gas, and changes to a class attribute will affect all instances of the class.

Dot Notation

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 hilfingers_car's color attribute:

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

Looks like we need to paint hilfingers_car!

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!

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

Awesome! But if you take a look at the paint method, it takes two parameters. So why don't we need to pass two arguments? Just like we've seen with __init__, all methods of a class have a self parameter to which Python automatically binds the instance that is calling that method. Here, hilfingers_car is bound to self so that the body of paint can access its attributes!

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

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

Notice that unlike when we painted Professor Hilfinger's car black, this time we had to pass in two arguments: one for self and one for color. This is because when you call a method using dot notation from an instance, Python knows what instance to automatically bind to self. However, when you call a method using dot notation from the class, Python doesn't know which instance of Car we want to paint, so we have to pass that in as well.

Inheritance

Professor Hilfinger's red Tesla is pretty cool, but it has to sit in traffic. 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)

Wow! The truck may be big, but the source code is tiny! Let's make sure that the truck still does what we expect it to do. Let's create a new instance of Professor Hilfinger's monster truck:

>>> hilfingers_truck = MonsterTruck('Monster Truck', 'XXL')

Does it behave as you would expect a Car to? Can you still paint it? Is it even drivable?

Well, 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.

>>> hilfingers_car.size
'Tiny'
>>> hilfingers_truck.size
'Monster'

Wow, what a difference in size! This is because the class attribute size of MonsterTruck overrides the size class attribute of Car, so all MonsterTruck instances are 'Monster'-sized.

In addition, the drive method in MonsterTruck overrides the one in Car. To show off all MonsterTruck instances, we defined a rev method specific to MonsterClass. Regular Cars cannot rev! Everything else -- the constructor __init__, paint, num_wheels, gas -- are inherited from Car.

Required Questions

WWPD

Q1: Using the Car class

Here are the full definitions of the Car and MonsterTruck classes 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 self.make + ' ' + self.model + ' 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 self.make + ' ' + self.model + ' gas level: ' + str(self.gas)

class MonsterTruck(Car):
    size = 'Monster'

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

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

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

python3 ok -q car -u
python3 ok -q food_truck -u

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

>>> hilfingers_car = Car('Tesla', 'Model S')
>>> hilfingers_car.color

______
'No color yet. You need to paint me.'

>>> hilfingers_car.paint('black')

______
'Tesla Model S is now black'

>>> hilfingers_car.color

______
'black'
>>> hilfingers_car = Car('Tesla', 'Model S')
>>> hilfingers_truck = MonsterTruck('Monster Truck', 'XXL')
>>> hilfingers_car.size

______
'Tiny'

>>> hilfingers_truck.size

______
'Monster'
>>> hilfingers_car = Car('Tesla', 'Model S')
>>> hilfingers_car.model

______
'Model S'

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

______
'Tesla Model S goes vroom!'

>>> hilfingers_car.drive()

______
'Tesla Model S cannot drive!'

>>> hilfingers_car.fill_gas()

______
'Tesla Model S gas level: 20'

>>> hilfingers_car.gas

______
20

>>> Car.gas

______
30
>>> Car.headlights

______
2

>>> hilfingers_car.headlights

______
2

>>> Car.headlights = 3
>>> hilfingers_car.headlights

______
3

>>> hilfingers_car.headlights = 2
>>> Car.headlights

______
3
>>> hilfingers_car.wheels = 2
>>> hilfingers_car.wheels

______
2

>>> Car.num_wheels

______
4

>>> hilfingers_car.drive()

______
'Tesla Model S cannot drive!'

>>> Car.drive()

______
Error (TypeError)

>>> Car.drive(hilfingers_car)

______
'Tesla Model S cannot drive!'

>>> MonsterTruck.drive(hilfingers_car)

______
Error (AttributeError)
>>> 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)
>>> class FoodTruck(MonsterTruck):
...    delicious = 'meh'
...    def serve(self):
...        if FoodTruck.size == 'delicious':
...            print('Yum!')
...        if self.food != 'Tacos':
...            return 'But no tacos...'
...        else:
...            return 'Mmm!'
>>> taco_truck = FoodTruck('Tacos', 'Truck')
>>> taco_truck.food = 'Guacamole'
>>> taco_truck.serve()

______
'But no tacos...'

>>> taco_truck.food = taco_truck.make
>>> FoodTruck.size = taco_truck.delicious
>>> taco_truck.serve()

______
'Mmm!'

>>> taco_truck.size = 'delicious'
>>> taco_truck.serve()

______
'Mmm!'

>>> FoodTruck.pop_tire()

______
Error

>>> FoodTruck.pop_tire(taco_truck)

______
Nothing

>>> taco_truck.drive()

______
Vroom! This Monster Truck is huge!
'Tacos Truck cannot drive!'

Adventure Game!

In the next part of this lab, we will be implementing a text-based adventure game! You can start the game by typing:

python3 adventure.py

To exit the game and return to the command line, type Ctrl-C or Ctrl-D.

Q2: Who am I?

It is time for you to enter a world of adventure! First, you need to create yourself as a Player object in data.py. Take a look at the Player class in classes.py and create a Player object at the bottom of data.py.

The Player constructor takes two arguments:

  • name should be your preferred name (as a string)
  • the starting place

Your Player should start at sather_gate.

# Player:
# The Player should start at sather_gate.

me = None

me = Player('Squirrel McSquirrelFace', sather_gate)

Use Ok to test your code:

python3 ok -q me

Once you've created your player, you can start the adventure game:

python3 adventure.py

You will see the following output (though the commands may be in a different order):

Welcome to the adventure game!

It's a bright sunny day.
You are a bright and eager CS 61A student named [your name],
wandering around Berkeley campus looking for some snacks
before the study party.

Let's go to FSM (Free Speech Movement Cafe)
and see what we can find there!

There are 7 possible commands:
    talk to [character]
    unlock [place]
    help
    take [thing]
    go to [place]
    look
    check backpack

adventure>

All you're able to do right now is look around, but let's change that!

Q3: Where do I go?

First, we need to be able to go to places. If you try the go to command, you'll notice it doesn't do anything.

In classes.py, complete the go_to method in the Player class by updating your place attribute to destination_place if it is not locked, and printing the name of the current place.

    def go_to(self, location):
        """Go to a location if it's among the exits of player's current place.

        >>> sather_gate = Place('Sather Gate', 'Sather Gate', [], [])
        >>> gbc = Place('GBC', 'Golden Bear Cafe', [], [])
        >>> sather_gate.add_exits([gbc])
        >>> sather_gate.locked = True
        >>> gbc.add_exits([sather_gate])
        >>> me = Player('player', sather_gate)
        >>> me.go_to('GBC')
        You are at GBC
        >>> me.place is gbc
        True
        >>> me.place.name
        'GBC'
        >>> me.go_to('GBC')
        Can't go to GBC from GBC.
        Try looking around to see where to go.
        You are at GBC
        >>> me.go_to('Sather Gate')
        Sather Gate is locked! Go look for a key to unlock it
        You are at GBC
        """
        destination_place = self.place.get_neighbor(location)
        if destination_place.locked:
            print(destination_place.name, 'is locked! Go look for a key to unlock it')

        "*** YOUR CODE HERE ***"

        else:
            self.place = destination_place
        print('You are at', self.place.name)

Use Ok to test your code:

python3 ok -q Player.go_to

After you complete this problem, you'll be able to move to different places and look around there. To complete the functionality of the game, including talking to characters and picking up items, finish Questions 5-8 under the Optional Questions section!

Optional Questions

Nonlocal Practice

Q4: Vending Machine

Implement the function vending_machine, which takes in a sequence of snacks (as strings) and returns a zero-argument function. This zero-argument function will cycle through the sequence of snacks, returning one element from the sequence in order.

def vending_machine(snacks):
    """Cycles through sequence of snacks.

    >>> vender = vending_machine(('chips', 'chocolate', 'popcorn'))
    >>> vender()
    'chips'
    >>> vender()
    'chocolate'
    >>> vender()
    'popcorn'
    >>> vender()
    'chips'
    >>> other = vending_machine(('brownie',))
    >>> other()
    'brownie'
    >>> vender()
    'chocolate'
    """

    "*** YOUR CODE HERE ***"

    index = 0
    def vender():
        nonlocal index
        result = snacks[index]
        index = (index + 1) % len(snacks)
        return result
    return vender

Use Ok to test your code:

python3 ok -q vending_machine

More Adventure!

Q5: How do I talk?

Now you can go wherever you want! Try going to Wheeler. There, you'll find Jerry. Try talking to him with the talk to command. This also doesn't work :(

Next, implement the talk_to method in Player. talk_to takes in the name of a Character object, and prints out the Character's response. Take a look at the doctest for more details.

Hint: talk_to takes in an argument person, which is a string. The characters instance attribute in self.place is a dictionary mapping Character names (strings) to Character objects.

Once you've got the Character object, what method in the Character class will make them talk?

def talk_to(self, person):
    """Talk to person if person is at player's current place.

    >>> jerry = Character('Jerry', 'I am not the Jerry you are looking for.')
    >>> wheeler = Place('Wheeler', 'You are at Wheeler', [jerry], [])
    >>> me = Player('player', wheeler)
    >>> me.talk_to(jerry)
    Person has to be a string.
    >>> me.talk_to('Jerry')
    Jerry says: I am not the Jerry you are looking for.
    >>> me.talk_to('Tiffany')
    Tiffany is not here.
    """
    if type(person) != str:
        print('Person has to be a string.')

    "*** YOUR CODE HERE ***"

    elif person not in self.place.characters:
        print(person, 'is not here.')
    else:
        print(person,'says:', self.place.characters[person].talk())

Use Ok to test your code:

python3 ok -q Player.talk_to

Q6: How do I take items?

Now you can talk to people in adventure world! To make it even better, let's implement the take method in the Player class so you can put things in your backpack. Currently, you don't have a backpack, so let's create an instance variable backpack and initialize it to an empty list.

After you've initialized your empty backpack, implement take, which takes in a name of an object as a name, checks if the Place you are at contains the Thing object corresponding to the input name, and then puts it into your backpack. Take a look at the doctests for more details.

Hint: the things instance attribute in the Place class is a dictionary that maps Thing names (strings) to Thing objects.

The take method in the Place class will also come in handy.

def take(self, thing):
    """Take a thing if thing is at player's current place

    >>> lemon = Thing('Lemon', 'A lemon-looking lemon')
    >>> gbc = Place('GBC', 'You are at Golden Bear Cafe', [], [lemon])
    >>> me = Player('Player', gbc)
    >>> me.backpack
    []
    >>> me.take(lemon)
    Thing should be a string.
    >>> me.take('orange')
    orange is not here.
    >>> me.take('Lemon')
    Player takes the Lemon
    >>> me.take('Lemon')
    Lemon is not here.
    >>> isinstance(me.backpack[0], Thing)
    True
    >>> len(me.backpack)
    1
    """
    if type(thing) != str:
        print('Thing should be a string.')

    "*** YOUR CODE HERE ***"

    elif thing not in self.place.things:
        print(thing, 'is not here.')
    else:
        taken = self.place.take(thing)
        print(self.name, 'takes the', taken.name)
        self.backpack.append(taken)

Use Ok to test your code:

python3 ok -q Player.take

Q7: No door can hold us back!

FSM is locked! There's no way for us to get in, and you're getting pretty desperate for that sweet, delicious, caffeinated nectar of the gods.

We'll need to do two things in order to get into FSM and get our caffeine fix.

Firstly, define a new class Key, that is a subclass of Thing, but overwrites the use method to unlock the door to FSM.

Hint 1: Refer back to the MonsterTruck example if you need a refresher on how to define a subclass and overwrite methods. Make sure you define Key after you've defined Thing.

Hint 2: Place has a locked instance attribute that you may need to change.

class Thing(object):
    def __init__(self, name, description):
        self.name = name
        self.description = description

    def use(self, place):
        print("You can't use a {0} here".format(self.name))


""" Implement Key here! """

class Key(Thing):
    def use(self, place):
        if place.locked:
            place.locked = False
            print(place.name, 'is now unlocked!')
        else:
            print(place.name, 'is already unlocked!')

You'll also need to finish the implementation of unlock in Player. It takes a string place that you want to unlock, and if you have a key, call the key's use method to unlock the place. If you have no key, then the method should print that the place can't be unlocked without a key.

You'll need to implement both Key and unlock for the doctests to pass.

def unlock(self, place):
    """If player has a key, unlock a locked neighboring place.

    >>> key = Key('SkeletonKey', 'A Key to unlock all doors.')
    >>> gbc = Place('GBC', 'You are at Golden Bear Cafe', [], [key])
    >>> fsm = Place('FSM', 'Home of the nectar of the gods', [], [])
    >>> gbc.add_exits([fsm])
    >>> fsm.locked = True
    >>> me = Player('Player', gbc)
    >>> me.unlock(fsm)
    Place must be a string
    >>> me.go_to('FSM')
    FSM is locked! Go look for a key to unlock it
    You are at GBC
    >>> me.unlock(fsm)
    Place must be a string
    >>> me.unlock('FSM')
    FSM can't be unlocked without a key!
    >>> me.take('SkeletonKey')
    Player takes the SkeletonKey
    >>> me.unlock('FSM')
    FSM is now unlocked!
    >>> me.unlock('FSM')
    FSM is already unlocked!
    >>> me.go_to('FSM')
    You are at FSM
    """
    if type(place) != str:
        print("Place must be a string")
        return
    key = None
    for item in self.backpack:
        if type(item) == Key:
            key = item

    "*** YOUR CODE HERE ***"

    if not key:
        print(place, "can't be unlocked without a key!")
    else:
        place_to_unlock = self.place.get_neighbor(place)
        key.use(place_to_unlock)

Use Ok to test your code:

python3 ok -q Player.unlock

Q8: Win the game!

Good job! Now you can explore around campus and try to win the game. Talk to the people at different places in order to get hints. Can you save the day and make it to the 61A project party in time?

python3 adventure.py

Enjoy!