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:
- Default constructer.
- 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.
- getattr(obj,name,default): This built-in function provides access to the attribute of the object.
- delattr(obj, name): This function is used to delete a particular class attribute.
- 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.
- 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.
- __dict__: This attributes returns the dictionary that contains some information concerning the class namespace.
- __name__: This attribute is used to returns the current class name.
- __doc__: This attribute carries a string, which has the current class documentation.
- __module__: We can access the module in which the class is defined.
- __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.