Constructors – default and parameterized

A constructor is a special kind of method which is used for initializing the instance variables during object creation. In this guide, we will see what is a constructor, types of it and how to use them in the python programming with examples.

1. What is a Constructor in Python?

Constructor is used for initializing the instance members when we create the object of a class.

For example:
Here we have a instance variable num which we are initializing in the constructor. The constructor is being invoked when we create the object of the class (obj in the following example).

class DemoClass:
    # constructor
    def __init__(self):
        # initializing instance variable
        self.num=100

    # a method
    def read_number(self):
        print(self.num)


# creating object of the class. This invokes constructor
obj = DemoClass()

# calling the instance method using the object obj
obj.read_number()

100

1.1 Syntax of constructor declaration

As we have seen in the above example that a constructor always has a name init and the name init is prefixed and suffixed with a double underscore(__). We declare a constructor using def keyword, just like methods.

2.1 Python – default constructor example

Note: An object cannot be created if we don’t have a constructor in our program. This is why when we do not declare a constructor in our program, python does it for us. Lets have a look at the example below.

In this example, we do not have a constructor but still we are able to create an object for the class. This is because there is a default constructor implicitly injected by python during program compilation, this is an empty default constructor that looks like this:

def __init__(self):
    # no body, does nothing.

Source Code:

 
class DemoClass:
    num = 101

    # a method
    def read_number(self):
        print(self.num)


# creating object of the class
obj = DemoClass()

# calling the instance method using the object obj
obj.read_number()

Output

101

 
class DemoClass:
    num = 101

    # non-parameterized constructor
    def __init__(self):
        self.num = 999

    # a method
    def read_number(self):
        print(self.num)


# creating object of the class
obj = DemoClass()

# calling the instance method using the object obj
obj.read_number()

Output

999

 
class DemoClass:
    num = 101

    # parameterized constructor
    def __init__(self, data):
        self.num = data

    # a method
    def read_number(self):
        print(self.num)


# creating object of the class
# this will invoke parameterized constructor
obj = DemoClass(55)

# calling the instance method using the object obj
obj.read_number()

# creating another object of the class
obj2 = DemoClass(66)

# calling the instance method using the object obj
obj2.read_number()

Output

55
66