Attributes

Accessing Attribute Values

Instance attributes are accessed using . between the instance and the attribute name.

Attribute values can be used anywhere a normal Python expression is valid.

Example

In the example below we create an account for Bruce Wayne, then print out the balance using batman_account.balance.

class Account:

    def __init__(self, bsb_number, account_number, balance, holder_name):
        self.bsb_number = bsb_number
        self.account_number = account_number
        self.balance = balance
        self.holder_name = holder_name


batman_account = Account("689716", "62228626", 19.39, "Bruce Wayne")

print(batman_account.balance)

Setting Attribute Values

To set the values of an attribute we use the syntax

instance.attribute = new_value

Example - Constructor

When we write self.attribute = new_value in the constructor, we are setting the instance attribute as shown in the example below.

class Account:

    def __init__(self, bsb_number, account_number, balance, holder_name):
        self.bsb_number = bsb_number
        self.account_number = account_number
        self.balance = balance
        self.holder_name = holder_name

Example - Overwriting a Value

Note

Instances and their attributes are mutable. This means we can overwrite their value after they are first set.

In the example below, the balance attribute gets overwritten with a new value.

class Account:

    def __init__(self, bsb_number, account_number, balance, holder_name):
        self.bsb_number = bsb_number
        self.account_number = account_number
        self.balance = balance
        self.holder_name = holder_name


batman_account = Account("689716", "62228626", 19.39, "Bruce Wayne")

batman_account.balance = 24.89

print(batman_account.balance)

Question 1

What would shape look like after running the following code?

class Rectangle:

    def __init__(self, height, width, colour):
        self.height = height
        self.width = width
        self.colour = colour

shape = Rectangle(3, 3, "blue")
shape.height = 6

Solution

Solution is locked

Question 2

Instances and their attributes are mutable. What does this mean?

1. It means that instances cannot have attributes.

2. It means that the attributes of an instance can only be the specified default values.

3. It means that all instances of a class must have the same attribute values.

4. It means that after the instance of a class is created, the instance and its attributes can still be changed.

Solution

Solution is locked

Code Challenge: Check the Fuel

Before takeoff, you’ll need to check how much fuel is on board the ship. The Spaceship class now has two attributes: name and fuel_level.

In this exercise create a new ship and print its name and the amount of fuel remaining.

Instructions

1. Create an instance of Spaceship named "Enterprise" with a fuel level of 50.

2. Access the name and fuel_level attributes from the instance.

3. Print both values.

Example Output

Spaceship Name: Enterprise
Fuel Level: 50

Here is some code for you to start with:

class Spaceship:
    def __init__(self, name, fuel_level):
        self.name = name
        self.fuel_level = fuel_level

Solution

Solution is locked

Code Challenge: Refuel

It looks like there isn’t sufficient fuel level aboard the spaceship. After fueling up all the way, set the fuel level to 100.

Instructions

1. Create an instance of Spaceship named "Enterprise" with a fuel level of 50.

2. Print the spaceship name and fuel level.

3. Afterwards set the fuel_level to 100.

4. Print the updated fuel level.

Example Output

Spaceship Name: Enterprise
Fuel Level: 50
Updated Fuel Level: 100

Here is some code for you to start with:

class Spaceship:
    def __init__(self, name, fuel_level):
        self.name = name
        self.fuel_level = fuel_level

Solution

Solution is locked

Code Challenge: Refuel by Weight

When we create a new spaceship it shouldn’t have any fuel on board. It’s also good practice measure fuel by weight rather than by percentage because calculations for liftoff use weight of fuel as a parameter.

Requirements

• Modify the Spaceship class so that it tracks fuel through two attributes: fuel_weight and fuel_capacity.

• The fuel_weight must always be zero when the spaceship is created

• The fuel_capacity can be set by the creator of the spaceship

Instructions

1. Add fuel_capacity as a parameter to __init__

2. Inside __init__:

   • Set the fuel_capacity to the parameter value

   • Set the fuel_weight to zero

3. Create an instance of Spaceship named "Enterprise" with a fuel capacity of 10000.

4. Afterwards set the fuel_weight to 5000.

5. Print the updated fuel remaining.

Example Output

Note

You can calculate the % of fuel remaining by dividing fuel weight by the capacity.

Fuel Remaining: 5000/10000 (50.00%)

Here is some code for you to start with:

class Spaceship:
    def __init__(self, name):
        self.name = name

Solution

Solution is locked