Solutions: You can find the file with solutions for all questions here.

Question 1: Account

There are several things wrong with the following code! Debug the Account class to satisfy the docstring.

class Account(object):
    """A bank account that allows deposits and withdrawals.

    >>> sophia_account = Account('Sophia')
    >>> sophia_account.deposit(1000000)   # depositing my paycheck for the week
    1000000
    >>> sophia_account.transactions
    [('deposit', 1000000)]
    >>> sophia_account.withdraw(100)      # buying dinner
    999900
    >>> sophia_account.transactions
    [('deposit', 1000000), ('withdraw', 100)]
    """

    interest = 0.02
    balance = 1000

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

    def deposit(self, amount):
        """Increase the account balance by amount and return the
        new balance.
        """
        self.transactions.append(('deposit', amount))
        Account.balance = self.balance + amount
        return self.balance

    def withdraw(self, amount):
        """Decrease the account balance by amount and return the
        new balance.
        """
        self.transactions.append(('withdraw', amount))
        if amount > self.balance:
            return 'Insufficient funds'
        self.balance = Account.balance - amount
        return Account.balance

Use OK to test your code:

python3 ok -q Account

Question 2: Arr88

In lab you created the T88ble, now you will create arr88, which are similar to numpy arrays from Data 8.

Complete the __len__, and item functions according to the docstrings.

__len__ is a special attribute, like __init__ that allows us to call len on our Arr88s to get their length!

    def __len__(self):
        """ Return the length of the Arr88

        >>> arr88 = Arr88([1, 2, 3])
        >>> len(arr88)
        3
        >>> arr88 = Arr88([1, 2, 3, 4])
        >>> len(arr88)
        4
        """
        return len(self._values)

    def item(self, i):
        """
        Get the item of the Arr88 at index i
        >>> arr88 = Arr88([1, 2, 3])
        >>> arr88.item(1)
        2
        >>> arr88.item(0)
        1
        """
        return self._values[i]

Use OK to test your code:

python3 ok -q Arr88.__len__

Use OK to test your code:

python3 ok -q Arr88.item

Complete the __add__, __mul__, and negate functions according to the docstrings.

Keep an eye out for which functions mutate the Arr88 and which don't!

__add__ and __mul__ are also special attributes, like __init__ and __len__, that allow us to use + and * on our Arr88s to add/multiply them componentwise!

    def __add__(self, arr88):
        """ Add two Arr88s of the same length element by element

        >>> arr88a = Arr88([1, 2, 3])
        >>> arr88b = Arr88([4, 5, 6])
        >>> arr88a + arr88b
        Arr88([5, 7, 9])
        >>> arr88a # We aren't mutating arr88a
        Arr88([1, 2, 3])
        >>> arr88a = Arr88(['He', 'Wor', '!'])
        >>> arr88b = Arr88(['llo', 'ld', ''])
        >>> arr88a + arr88b
        Arr88(['Hello', 'World', '!'])
        """
        # Checks that the lengths are the same
        assert len(self) == len(arr88), "Arr88's of different len"
        return Arr88([a+b for a,b in zip(self._values, arr88._values)])

    def __mul__(self, arr88):
        """ Multiply two Arr88s of the same length componentwise

        >>> arr88a = Arr88([1, 2, 3])
        >>> arr88b = Arr88([4, 5, 6])
        >>> arr88a * arr88b
        Arr88([4, 10, 18])
        >>> arr88a # We aren't mutating arr88a
        Arr88([1, 2, 3])
        >>> arr88a = Arr88(['Na', 'Batman', '!'])
        >>> arr88b = Arr88([10, 1, 5])
        >>> arr88a * arr88b
        Arr88(['NaNaNaNaNaNaNaNaNaNa', 'Batman', '!!!!!'])
        """
        # Checks that the lengths are the same
        assert len(self) == len(arr88), "Arr88's of different len"
        return Arr88([a*b for a,b in zip(self._values, arr88._values)])

    def negate(self):
        """Negate an Arr88 with mutation

        >>> arr88a = Arr88([1, 2, 3])
        >>> arr88b = Arr88([4.0, -5.5, 0.0])
        >>> arr88a.negate()
        >>> arr88a
        Arr88([-1, -2, -3])
        >>> arr88b.negate()
        >>> arr88b
        Arr88([-4.0, 5.5, -0.0])
        """
        self._values = [-a for a in self._values]

Use OK to test your code:

python3 ok -q Arr88.__add__

Use OK to test your code:

python3 ok -q Arr88.__mul__

Use OK to test your code:

python3 ok -q Arr88.negate

Complete the apply function that returns a new list with the function applied to every element.

    def apply(self, func):
        """ Apply a function to an Arr88

        >>> arr88a = Arr88([1, 2, 3])
        >>> arr88a.apply(lambda x : x * x)
        Arr88([1, 4, 9])
        >>> arr88a # We aren't mutating arr88a
        Arr88([1, 2, 3])
        >>> arr88b = Arr88([lambda x: x, lambda x: x + 1, lambda x: x + 2])
        >>> arr88b.apply(lambda f: f(1))
        Arr88([1, 2, 3])
        """
        return Arr88([func(a) for a in self._values])

Use OK to test your code:

python3 ok -q Arr88.apply

Question 3: Vending Machine

Create a class called VendingMachine that represents a vending machine for some product. A VendingMachine object returns strings describing its interactions. See the doctest below for examples:

Here's a quick explanation of some of the functions you need to implement.

restock should update the stock and return the current stock.

deposit should add money to the balance and return the current balance, unless the stock is zero, then it should inform the user the stock is zero and return the money.

vend should either tell you how much more money needs to be deposited to buy a product, or sell you a product and return the change, or let you know the machine is out of stock.

Make sure your outputs match the doctest exactly!

Hint: .format() can help you format your strings. Here's an example:

>>> name = "Alex"
>>> 'My name is {}'.format(name)
'My name is Alex'
class VendingMachine:
    """A vending machine that vends some product for some price.

    >>> v = VendingMachine('candy', 10)
    >>> v.vend()
    'Machine is out of stock.'
    >>> v.restock(2)
    'Current candy stock: 2'
    >>> v.vend()
    'You must deposit $10 more.'
    >>> v.deposit(7)
    'Current balance: $7'
    >>> v.vend()
    'You must deposit $3 more.'
    >>> v.deposit(5)
    'Current balance: $12'
    >>> v.vend()
    'Here is your candy and $2 change.'
    >>> v.deposit(10)
    'Current balance: $10'
    >>> v.vend()
    'Here is your candy.'
    >>> v.deposit(15)
    'Machine is out of stock. Here is your $15.'
    """
    def __init__(self, product, price):
        self.product = product
        self.price = price
        self.stock = 0
        self.balance = 0

    def restock(self, n):
        self.stock += n
        return 'Current '+self.product+' stock: '+str(self.stock)

    def deposit(self, n):
        if self.stock == 0:
            return 'Machine is out of stock. Here is your $'+str(n)+'.'
        self.balance += n
        return 'Current balance: $'+str(self.balance)

    def vend(self):
        if self.stock == 0:
            return 'Machine is out of stock.'
        difference = self.price - self.balance
        if self.balance < self.price:
            return 'You must deposit $'+str(difference)+' more.'
        message = 'Here is your '+str(self.product)
        if difference != 0:
            message += ' and $'+str(-difference)+' change'
        self.balance = 0
        self.stock -= 1
        return message + '.'

Use OK to test your code:

python3 ok -q VendingMachine