Module 4 Lesson 4: Instance Attributes vs. Methods

We now know how to give our objects data (Attributes). But an object isn't just a container; it's a living part of your program that can do things. In this lesson, we’ll explore Methods—functions that belong to a specific object.

Lesson Overview

In this lesson, we will cover:

• Attributes: The "Nouns" (Data).
• Methods: The "Verbs" (Actions).
• The self in Methods: Accessing an object's data from its actions.
• Updating State: How methods can change attributes over time.

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1. Nouns vs. Verbs

If you were building a "Bank Account" in code:

• Attributes (Nouns): account_number, balance, owner_name.
• Methods (Verbs): deposit(), withdraw(), check_balance().

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2. Defining an Instance Method

Methods are defined inside a class just like normal functions, but with one difference: The first parameter must always be self.

class Dog:
    def __init__(self, name):
        self.name = name
    
    # This is an instance method
    def bark(self):
        print(f"{self.name} says: Woof! Woof!")

my_dog = Dog("Buddy")
my_dog.bark() # Output: Buddy says: Woof! Woof!

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Visualizing the Process

graph TD
    Start[Input] --> Process[Processing]
    Process --> Decision{Check}
    Decision -->|Success| End[Complete]
    Decision -->|Retry| Process

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3. Modifying Attributes via Methods

The most powerful thing a method can do is change the object's data. This is how we simulate real-world changes.

class Car:
    def __init__(self, brand, speed=0):
        self.brand = brand
        self.speed = speed
    
    def accelerate(self, amount):
        self.speed += amount
        print(f"The {self.brand} is now going {self.speed} mph.")

my_car = Car("Tesla")
my_car.accelerate(20) # Speed becomes 20
my_car.accelerate(30) # Speed becomes 50

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4. Why Use Methods instead of Changing Data Directly?

You could just write my_car.speed = 100. But using a method like accelerate() allows you to add Rules. For example:

• "A car cannot accelerate past 120 mph."
• "A car cannot accelerate if the engine is off."

By using methods, you ensure your objects follow the laws of your "digital world."

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Practice Exercise: The Fitness Tracker

Create a file named fitness.py.

1. Define a class User.
2. In __init__, set name and steps_taken (defaulting to 0).
3. Add a method walk(steps) that adds to the total steps and prints: "[Name] walked [X] steps today!"
4. Add a method reset_steps() that sets the steps back to 0.
5. Create a user object, walk twice, and then reset.

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Quick Knowledge Check

1. What is the first parameter of every instance method?
2. How is a method different from a regular function?
3. Can a method access the object’s attributes? If so, how?
4. Give an example of a "rule" you might put inside a withdraw() method for a bank account.

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Key Takeaways

• Attributes store data (state); Methods define behavior.
• Methods use self to interact with the object's own attributes.
• Methods can update attributes dynamically.
• Methods allow you to implement logic and rules for your data.

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What’s Next?

We’ve seen that we can change attributes directly (my_car.speed = 500). But what if we want to hide some data so it can only be changed by our rules? In Lesson 5, we’ll learn about Encapsulation and Data Hiding!