Module 4 Lesson 3: The __init__ Method

In the last lesson, we created objects and manually added attributes like dog.name = "Buddy". This is fine for one object, but if you have 10,000 objects, it’s impossible. Python solves this with a special "Constructor" method called __init__.

Lesson Overview

In this lesson, we will cover:

• The Constructor: What __init__ does.
• The self Keyword: Connecting data to a specific object.
• Parameters in Classes: Passing data during creation.
• Real-world Example: Building a more efficient User class.

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1. What is __init__?

The name __init__ is short for initialize. It is a special function that runs automatically the moment you create a new object from a class. Think of it as the "birth" of the object where its basic traits are determined.

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2. Meet the self Keyword

Inside a class, we use self to refer to the current object being worked on. It’s like the object saying, "Hey Python, this is my name, not some other dog’s name."

class Dog:
    def __init__(self, name, breed):
        self.name = name   # Store the input 'name' into the object's trait
        self.breed = breed # Store the input 'breed' into the object's trait

# Now we pass the data inside the parentheses!
my_dog = Dog("Buddy", "Golden Retriever")
your_dog = Dog("Max", "Husky")

print(my_dog.name)   # Output: Buddy
print(your_dog.name) # Output: Max

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3. How it Works (Under the Hood)

When you run my_dog = Dog("Buddy", "Golden Retriever"):

1. Python creates an empty object in memory.
2. Python automatically calls __init__.
3. The word self becomes a label for that new empty object.
4. The data ("Buddy", "Golden Retriever") is assigned to the self labels (self.name, self.breed).
5. The finished object is returned to the variable my_dog.

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4. Default Values in __init__

Just like normal functions, you can have default values in your constructors.

class User:
    def __init__(self, username, role="Member"):
        self.username = username
        self.role = role

admin = User("MasterDev", "Admin")
guest = User("Anonymous") # role defaults to "Member"

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Practice Exercise: The Smartphone Maker

Create a file named smartphones.py.

1. Define a class Phone.
2. Add an __init__ method that takes brand, model, and storage.
3. Inside __init__, also create an attribute battery_level that always starts at 100 (does not need to be a parameter!).
4. Create two phone objects with different brands and models.
5. Print out their details and their initial battery levels.

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Quick Knowledge Check

1. What does __init__ stand for?
2. When does the __init__ method run?
3. In your own words, what does the self keyword represent?
4. True or False: You have to manually call my_object.__init__() to make it work.

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Key Takeaways

• __init__ is the constructor method in Python.
• It is used to set initial data (attributes) for your objects.
• self is a reference to the current instance of the class.
• Data passed during instantiation goes directly into the __init__ parameters.

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What’s Next?

We know how to store data in objects. Now let's learn how to make them do something! In Lesson 4, we’ll explore Instance Methods—functions that belong specifically to an object!