Python Constructor

Introduction

A constructor is defined as the special kind of function or method that is used for instance variables initialization during the creation of an object of a class.

The constructor's task is to assign values(initialize) to the instance members of the class when a class object is created. 

Creating a constructor:

The __init__ method or function is used for creating a constructor of the class and it is also used when an object is created.

As a first argument, it takes the self-keyword which allows accessing the methods and attributes of the class. Initializing the attributes of a class is the most common use of constructor in Python.

Any number of arguments can be passed at the time of creating an object of a class, once the __init__() function is defined. Each class is required to have a constructor, still if it just relies on the default constructor.

Syntax:

def __init__(self):

    # body

Example:

class DummyClass:

    # constructor

    def __init__(self):

        # initializing instance variable

        self.num=200

    # a method

    def read_number(self):

        print(self.num)

# creating object of the class

obj = DummyClass()

# using obj to call the instance method 

obj.read_number()

Output:

200

Explanation: In the above example, we have defined how to create a constructor in Python.

Constructors are divided into two types:

1. Default constructer.
2. Parametrized constructer.

1. Non-parameterized constructor:

The non-parameterized constructor can be defined as a simple constructor or default constructor.

• It doesn’t accept any arguments. 
• This constructor has no parameter exist so, it is called non parametrized constructor or default constructor. 
• Its definition has only one argument and it works as a reference to the instance being constructed. 
• It takes only self-reference.
• When each class object requires to be initialized with the same values, this constructor is very useful at that time.

Example 1:

class JavaTpoint:

    # default constructor

    def __init__(self):

        self.Java = "JavaTpoint"

    # a method for printing data members

    def print_Java(self):

        print(self.Java)

# creating object of the class

obj = JavaTpoint()

# calling the instance method using the object obj

obj.print_Java()

Output:

JavaTpoint

Example 2:

class Person:

#Constructor - non parameterized

def __init__(self):

print("This constructor is non parametrized")

def show(self,name):

print("Hello",name)

person = Person()

person.show("EDDY")

Output:

This constructor is non parametrized

Hello EDDY

2. Parameterized Constructor:

constructor which contains parameters is referred to as a parameterized constructor.

• The parameterized constructor uses its first argument as a reference to the instance being created called as self and the Programmer provides the rest of the arguments. 
• The parameterized constructor has many parameters onward with the self. 
• Parameterized Constructors are the constructor with arguments. 
• A parameterized constructor can have several arguments.

Example 1:

class Person:

#parameterized constructer

def __init__(self, name):

print("This constructor is parametrized")

self.name = name

def show(self):

print("Hello",self.name)

person = Person("EDDY")

person.show()

Output:

This constructor is parametrized

Hello EDDY

Example 2:

Class Substraction:

    first = 0

    second = 0

    answer = 0

    # parameterized constructor

    def __init__(self, f, s):

        self.first = f

        self.second = s

    def display(self):

        print("First number = " + str(self.first))

        print("Second number = " + str(self.second))

        print("Substraction of number = " + str(self.answer))

    def calculate(self):

        self.answer = self.first - self.second

# creating object of the class

# this will invoke parameterized constructor

obj = Substraction(5000, 3000)

# perform substraction

obj.calculate()

# display result

obj.display()

Output:

First number = 5000

Second number = 3000

Substraction of numbers = 2000

Multiple constructors in a single class

Python does not provide constructor overloading. let's see what happens when we provide it, two constructors.

Example:

class Demo:

 def __init__(self):

 print("First constructor")

 def __init__(self):

 print("Second constructor")

    d= Demo()

Output:

Second constructor

Explanation: In the above code, the second constructor is called by the object d whether both have the same specification. The first function is not accessible by the object d. 

Generally, if the class has more than one constructor, then always the last constructor is called by the object of the class. 

When the constructor is not declared

When the constructor is not defined Python provides a default constructor and it instantiates the object.

Example:

class demo:

def show(self):

print("This is a default constructor :)")

>>> d=demo()

>>> d.show()

Output:

This is a default constructor 

Built-in class functions in Python

Here, Various built-in functions in the class exist in Python. Following are the built-in class functions in Python.

1. getattr(obj,name,default): This built-in function provides access to the attribute of the object.
2. delattr(obj, name): This function is used to delete a particular class attribute.
3. setattr(obj, name,value): This function is used, when there is a need to set a specific value to a specific attribute of an object.
4. hasattr(obj, name): This function returns true if a particular object carries some specific attribute.

Example:

class Employee:

def __init__(self,name,id,age):

self.name = name;

self.id = id;

self.age = age

#creates the object of the class Employee

emp = Employee("EDDY",121,33)

#prints the attribute name of the object s

print(getattr(emp,'name'))

# reset the value of attribute age to 34

setattr(emp,"age",34)

# prints the modified value of age

print(getattr(emp,'age'))

# prints true if the Employee contains the attribute with name id

print(hasattr(emp,'id'))

# deletes the attribute age

delattr(emp,'age')

# this will give an error since the attribute age has been deleted

print(emp.age)

Output:

EDDY

34

True

AttributeError: 'Employee' object has no attribute 'age'

Built-in class attributes in Python

Python also contains built-in class attributes along with built-in class functions. Below are some built-in class attributes in Python.

1. __dict__: This attributes returns the dictionary that contains some information concerning the class namespace.  
2. __name__: This attribute is used to returns the current class name.
3. __doc__: This attribute carries a string, which has the current class documentation.
4. __module__: We can access the module in which the class is defined.
5. __bases__: This attribute contains a tuple in which all base classes are included.

Example:

class Employee:    

    def __init__(self,name,id,age):    

        self.name = name;    

        self.id = id;    

        self.age = age    

    def display_details(self):    

        print("Name:%s, ID:%d, age:%d"%(self.name,self.id))    

emp = Employee("EDDY",121,33)   

print(s.__doc__)    

print(s.__dict__)    

print(s.__module__)  

Output:

None

{'name': 'EDDY', 'id': 121, 'age': 33}

__main__

Conclusion

In this article, we have discussed constructors and types of constructors in Python such as parameterized and non-parameterized constructors. Also, We have seen different methods inside the constructors.