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.

Checkpoint: Creating patient vital signs instances

Concepts in Practice: Defining and using __init__() with parameters

Consider the example above.

Concepts in Practice \(\PageIndex{1}\): Defining and using __init__() with parameters

Suppose the programmer wanted to make blood pressure a required parameter in __init__(). Which is the correct __init__() method header?

1. def __init__(p_id, bp, self, tmp=-1.0, hr=-1, rr=-1):
2. def __init__(self, p_id, bp, tmp=-1.0, hr=-1, rr=-1):
3. def __init__(self, p_id, bp=[-1,-1], tmp=-1.0, hr=-1, rr=-1):

Answer

b. Removing the default parameter value makes bp a required parameter.

Concepts in Practice \(\PageIndex{2}\): Defining and using __init__() with parameters

Which is a correct call to __init__() to create an instance with p_id=5241?

1. patient_10 = Vitals(self, 5241)
2. patient_10 = Vitals(5241)
3. both

Answer

b. self is the first parameter in __init__() definition, not an explicit argument. self should not be included in the call.

Concepts in Practice \(\PageIndex{3}\): Defining and using __init__() with parameters

Suppose another __init__() definition is added after the first with the header as follows:

def __init__(self, p_id, bp, tmp, hr, rr)

What is the impact on the program?

1. no change.
2. Second __init__() definition produces an error.
3. First two __init__() calls produce an error.

Answer

c. The second definition overwrites the first as multiple explicit definitions of __init__() aren't allowed. __init__() requires all parameters, so the first two __init__() calls for patient_1 and patient_2 are incorrect.

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 11.1: 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)  

Concepts in Practice \(\PageIndex{4}\)

What does order_1 represent at the end of the program?

1. 8 oz coffee
2. 8 oz coffee with milk
3. 8 oz coffee with sugar

Answer

c. order_1 is originally a coffee without milk or sugar, but the call to change() adds sugar to the order by assigning order_1.with_sugar with True .

Concepts in Practice \(\PageIndex{5}\)

Why is change() an instance method?

1. can change attributes
2. has multiple parameters
3. can change an instance

Answer

c. An instance method's first parameter indicates the method can change an instance. change() allows a change in whether a coffee order has milk or sugar.

Concepts in Practice \(\PageIndex{6}\)

Can print_order() be unindented?

1. yes
2. no

Answer

b. Unindenting print_order() would produce an error. An instance method must be defined in the class.

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

Interactive Code

# 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()

Input

80
50

Answer

class VendingMachine:
def __init__(self, num):
self.count = num
self.max = num

def refill(self):
self.count = self.max
print('Refilled')
def sell(self, order):
self.count -= order
print('Sold:', order)
def print_stock(self):
print('Current stock:', self.count)

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()