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

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

Consider the example above.

1.

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):

def __init__(self, p_id, bp, tmp=-1.0, hr=-1, rr=-1):

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

2.

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

3.

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.

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)
  

4.

What does order_1 represent at the end of the program?

1. 8 oz coffee

8 oz coffee with milk

8 oz coffee with sugar

5.

Why is change() an instance method?

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

6.

Can print_order() be unindented?

1. yes
2. no

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
  

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