Online STEM and Robotics Club for Macau Peninsula Students

1. Introduction: Initializing Object Data Attributes

When you define a class template, its objects need data attributes to hold their information. If you do not assign initial values to an object's data attributes when it is created, those values might either remain unset or take arbitrary system defaults.

To avoid unpredictability, it is best practice to explicitly define default or initial values for an object’s attributes right at the moment of creation. If the user wants to update those values later during execution, they can easily modify them.

In Python, this initialization process is handled automatically by a special method known as the constructor.

2. What Is a Constructor and Why Is It Called __init__?

• The __init__ keyword: In Python, the constructor is written as __init__ with two leading underscores and two trailing underscores.
• The first method called: The constructor is called a “constructor” because its job is to construct and initialize the object. Whenever a new object instance is created from a class, the constructor is always the very first method that executes.
• Automatic execution: You never need to call obj.__init__() explicitly. When you type the class name followed by parentheses (e.g., Dog() or Car()), those parentheses represent the constructor call. Python automatically identifies the class name with parentheses and executes the __init__ method behind the scenes.

3. Example 1: Constructor with Built-In Default Values (The Dog Class)

In this pattern, the constructor assigns fixed default values directly inside its body without requiring external inputs at creation time:

class Dog:
    def __init__(self):
        # Setting a default attribute value during construction
        self.name = "Buddy"

    def bark(self):
        print(f"{self.name} says woof")

Execution Walkthrough

# Instantiating the dog object
my_dog = Dog()

# Calling the bark method
my_dog.bark()

Output:

Buddy says woof

• Writing my_dog = Dog() runs the __init__ method automatically.
• The constructor initializes self.name with the default string "Buddy".
• When my_dog.bark() is called, it outputs "Buddy says woof" using the value initialized during construction.
• If needed, the user can change this attribute value later.

4. Example 2: Parameterized Constructor with Input Arguments (The Car Class)

Rather than hardcoding default values for every object, you can pass input arguments directly into the __init__ constructor. This allows each object to be created with its own custom attribute values from the start:

class Car:
    # Constructor accepting input arguments for brand and color
    def __init__(self, brand, color):
        self.brand = brand
        self.color = color

Execution Walkthrough

You pass the specific arguments inside the parentheses when instantiating the object:

# 1. Creating car1 with brand "Toyota" and color "red"
car1 = Car("Toyota", "red")

# Printing car1 attributes
print("Car 1 Brand:", car1.brand)
print("Car 1 Color:", car1.color)

Output:

Car 1 Brand: Toyota
Car 1 Color: red

• When Car("Toyota", "red") runs, Python routes "Toyota" and "red" directly into the __init__ constructor’s parameters.
• The constructor assigns them internally (self.brand = brand and self.color = color), establishing those attributes for car1.

5. Modifying Attributes and Managing Multiple Instances

Attributes configured during construction remain fully mutable, and different instances maintain completely independent states:

# Modifying an attribute on car1
car1.color = "blue"

# 2. Creating an independent second object: car2
car2 = Car("Honda", "white")

# Inspecting both objects
print("Car 2:", car2.brand, car2.color)
print("Car 1 updated color:", car1.color)

Output:

Car 2: Honda white
Car 1 updated color: blue

• car2 is instantiated with its own unique data ("Honda" and "white").
• Modifying car1.color to "blue" updates only car1, leaving car2 unaffected.