Vending machine – Take 2: Using OOP

This tutorial revisits a previous example and uses object-oriented programming paradigms to represent the concepts related to a vending machine. We will see how classes and objects will help us better represent the "real world" and help us track state:

Design

Let's begin by designing our vending machine by modelling different objects. First, we have:

• Coins
• Products
• Vending Machine

We can consider these as abstract classes / concepts. In practise, we have specific coins like quarters, loonies & toonies. For products, we have chips, candy and drinks – these could be broken down further into specific products like "355ml Coca-Cola can". These are considered concrete classes which all share common properties to the abstracts coins and products.

We must also model our vending machine and define actions on it:

• + insert_coin(coin: Coin)
• + buy_product(product: type) -> Product
• + get_balance() -> int
• + get_change() -> List[Coin]

Models

Coins

Coin
value: int
label: str
str() -> str

The following classes inherit from the parent Coin class. Inheritance is achieved by providing parentheses around the class definition and providing the parent class(es) as arguments.

Nickel(Coin)
value	5
str()	'5¢'

Dime(Coin)
value	10
str()	'10¢'

Quarter(Coin)
value	25
str()	'25¢'

Loonie(Coin)
value	100
str()	'$1'

Toonie(Coin)
value	200
str()	'$2'

Products

Product
name: str
price: int
str() -> str

Chips(Product)
name	'Chips'
price	225
str() -> str	Chips: $2.25

Drink(Product)
name	'Drink'
price	275
str() -> str	Drink: $2.75

Candy(Product)
name	'Candy'
price	315
str()	Candy: $3.15

Vending Machine

VendingMachine
coins: List[Coin]
purchases: List[Product]
insert_coin(coin: Coin)
buy_product(product: type) -> Product
get_balance() -> int
get_change() -> List[Coin]

.insert_coin(coin: Coin)

• The coin parameter will accept any instance of the Coin class

• When the function insert_coin() is called, store the inserted coin a list on the object

.buy_product(product: type) -> Product

• The product argument may be one of the following types: Drink, Candy, or Chips, but be flexible enough to accept other Product types. Any other value should raise a ValueError exception. Note: these are the Product classes we've defined, and not their instances.

  E.g.

  machine = VendingMachine()
  machine.buy_product(products.Drink)  # Good: This is a type
  machine.buy_product(products.Drink())  # Bad: This is an instance

  This is the same difference as the type int and the instance int()

• If the vending machine balance is less than the cost of the product, a custom exception called InsufficientFunds should be raised.

• Upon successful purchase, an instance of the product should be returned, and, the purchase should be added to a list of purchases on the object.

.get_balance() -> int

• The get_balance function should return the sum of inserted coins minus the sum of the price of purchased products.

• Scenario: when no coins are inserted, and no purchases have been made, the balance should be zero.

• Scenario: given that two toonies are inserted into the machine and a candy bar was purchased, the method should return 75.

.get_change() -> List[Coin]

• If the vending machine balance is greater than zero, return a list of coins (can be any combination of 5, 10, 25, 100, 200) which will sum up to the balance.

• If for whatever reason the balance is not a multiple of 5, then the sum of coins returned should be rounded down to the nearest multiple of 5, and not exceed the balance.

• The coins returned should be the largest first, then the smallest.

• The list of inserted coins & purchased products should be cleared (get_balance should be zero)

• Scenarios:

  • When the balance is 0, no quarters should be returned
  • When the balance is 25, a quarter should be returned
  • When the balance is 400, two toonies should be returned
  • When the balance is 300, a toonie and loonie should be returned
  • When the balance is 265, a toonie, two quarters, a dime and a nickel should be returned
  • When the balance is 7, a nickel should be returned
  • When the balance is negative, nothing should be returned

Solving

• Begin by building the Coin classes, followed by the Product. Starter tests have already been provided for these classes.
• Start writing tests for the VendingMachine methods, in the order defined above.