11.3 Instance methods

Learning objectives

By the end of this section you should be able to

Create and implement __init__ with multiple parameters including default parameter values.
Describe what information an instance method has access to and can modify.

More about init

In Python, __init__ is the special method that creates instances. __init__ must have the calling instance, self, as the first parameter and can have any number of other parameters with or without default parameter values.

Instance methods

An instance method is used to access and modify instance attributes as well as class attributes. All methods shown so far, and most methods defined in a class definition, are instance methods.

Example 1 Instance methods are often used to get and set instance information

class ProductionCar:
  def __init__(self, make, model, year, max_mph = 0.0):
    self.make = make
    self.model = model
    self.year = year
    self.max_mph = max_mph

  def max_kmh(self):
    return self.max_mph * 1.609344

  def update_max(self, speed):
    self.max_mph = speed

car_1 = ProductionCar('McLaren', 'Speedtail', 2020) # car_1.max_mph is 0.0
car_1.update_max(250.0) # car_1.max_mph is 250.0
print(car_1.make, car_1.model, 'reaches', car_1.max_mph, 'mph (',
  car_1.max_kmh(), 'km/h)') # Prints McLaren Speedtail reaches 250.0 mph (402.336 km/h)

Concepts in Practice

CoffeeOrder instance methods

Consider the example below:

class CoffeeOrder:
  loc = 'Cafe Coffee'
  cls_id = 1

  def __init__(self, size=16, milk=False, sugar=False):
    self.order_id = CoffeeOrder.cls_id
    self.cup_size = size
    self.with_milk = milk
    self.with_sugar = sugar
    CoffeeOrder.cls_id += 1

  def change(self, milk, sugar):
    self.with_milk = milk
    self.with_sugar = sugar

  def print_order(self):
    print(CoffeeOrder.loc,'Order', self.order_id, ':', self.cup_size, 'oz')
    if self.with_milk:
      print('\twith milk')
    if self.with_sugar:
      print('\twith sugar')

order_1 = CoffeeOrder(8)
order_2 = CoffeeOrder(8, True, False)
order_1.change(False, True)

Try It

Creating a class for vending machines

Write a class, VendingMachine, as described below. Default values follow the attributes. Ex: count's default value is 0. Create a vending machine using a value read from input and call instance methods.

Instance attributes:

count: 0
max: 0

Methods:

__init__(num): initializes count and max with num parameter
refill: assigns count with max's value and prints "Refilled"
sell(order): assigns count with the value of count minus order and prints "Sold: [order]"
print_stock: prints "Current stock: [count]"

Given input:


  100
  25

The output is:


  Current stock: 100
  Sold: 25
  Current stock: 75
  Refilled
  Current stock: 100

# Define VendingMachine class 

stock = int(input())
vm = VendingMachine(stock)
vm.print_stock()
order_1 = int(input())
vm.sell(order_1)
vm.print_stock()
vm.refill()
vm.print_stock()

Adapted from Introduction to Python Programming by OpenStax (openstax.org), licensed under CC BY-NC-SA 4.0. Changes were made. License: CC-BY-NC-SA-4.0.