Constructors and Instance Methods

Initializing Objects

Yesterday, we created an empty object and added attributes manually. In professional code, we use a Constructor to set up an object’s data as soon as it is created.

The __init__ Method

In Python, the constructor is a special method called __init__ (short for initialization). It runs automatically when you create a new object.

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class Dog:
    def __init__(self, name, breed):
        self.name = name   # Assigning param to object attribute
        self.breed = breed
        print(f"{self.name} has been created!")

# Now we pass data during creation
my_dog = Dog("Rex", "German Shepherd")
friend_dog = Dog("Bella", "Poodle")

print(my_dog.name)
print(friend_dog.name)

Instance Methods

Methods can use the data stored in the object to perform specific tasks.

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class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def introduce(self):
        print(f"Hi, I'm {self.name} and I am {self.age} years old.")

    def human_years(self):
        return self.age * 7

rex = Dog("Rex", 3)
rex.introduce()
print(f"In human years, Rex is {rex.human_years()}")

Why use self?

Remember, self is how the method knows which object’s data to use. If you have two dogs, self.name ensures Rex says “Rex” and Bella says “Bella”.

Interactive Practice

Try building a Book class that takes title and author as arguments in the __init__ method. Add a method called get_description that returns a string like “Title by Author”.