Python - Class Constructor __init__ method

In this tutorial we will learn about the class __init__ method in Python.

We learned about classes and how to create objects using classes in the Python - Classes and Objects tutorial. Feel free to check that out.

The __init__ method

The __init__ method is a special method of a class.

It is also called the constructor method and it is called when we create (instantiate) an object of the class.

We use the __init__ method to initialise class attributes or call class methods.

In the following Python program we are creating the __init__ method inside the Awesome class.

# class
class Awesome:

    # the init method
    def __init__(self):
        print("Hello from the __init__ method.")

# object of the class
obj = Awesome()

The above code will print the following output.

Hello from the __init__ method.

We get the above output because as soon as we create an object of the class the __init__ method is automatically called.

Passing argument to the __init__ method

Like any other methods of a class we can also pass arguments to the __init__ method.

In the following Python program we are passing a string value to the __init__ method at the time of creating an object.

# class
class Awesome:

    # the init method
    def __init__(self, name):
        print("Hello from the __init__ method.")
        print("Name:", name)

# object of the class
obj = Awesome("Yusuf Shakeel")

The above code will give us the following output.

Hello from the __init__ method.
Name: Yusuf Shakeel

Calling other methods from the __init__ method

We can call other methods of the class from the __init__ method by using the self keyword.

# class
class Awesome:

    # the init method
    def __init__(self):
        print("Hello from the __init__ method.")

        # calling the class method
        self.greetings()

    # methods
    def greetings(self):
        print("Hello from the greetings() method.")

# object of the class
obj = Awesome()

The above code will print the following output.

Hello from the __init__ method.
Hello from the greetings() method.

Initialise class properties inside the __init__ method

We can create class properties that are specific to the class objects and initialise them inside the __init__ method.

In the following Python program we are creating Awesome class and initialising class property name inside the __init__ method.

# class
class Awesome:

    # class attribute common to all objects
    commonAttr = "Common Attribute"

    # the init method
    def __init__(self, name):

        # class attribute for the objects
        self.name = name

    # methods
    def greetings(self):
        print("Hello %s!" %self.name)

# object of the class
obj1 = Awesome("Tom")
obj2 = Awesome("Jerry")

# output
print("The common class attribute:")
print("obj1.commonAttr:", obj1.commonAttr)
print("obj2.commonAttr:", obj2.commonAttr)

print("Class attribute specific to object:")

print("obj1.greetings():")
obj1.greetings()

print("obj2.greetings():")
obj2.greetings()

The above code will print the following output.

The common class attribute:
obj1.commonAttr: Common Attribute
obj2.commonAttr: Common Attribute
Class attribute specific to object:
obj1.greetings():
Hello Tom!
obj2.greetings():
Hello Jerry!