Python Constructors

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Python Constructor

A constructor is a special type of method (function) which is used to initialize the instance members of the class.

In C++ or Java, the constructor has the same name as its class, but it treats constructor differently in Python. It is used to create an object.

Constructors can be of two types.

Parameterized Constructor
Non-parameterized Constructor

Constructor definition is executed when we create the object of this class. Constructors also verify that there are enough resources for the object to perform any start-up task.

Creating the constructor in python

In Python, the method the __init__() simulates the constructor of the class. This method is called when the class is instantiated. It accepts the self-keyword as a first argument which allows accessing the attributes or method of the class.

We can pass any number of arguments at the time of creating the class object, depending upon the __init__() definition. It is mostly used to initialize the class attributes. Every class must have a constructor, even if it simply relies on the default constructor.

Consider the following example to initialize the Employee class attributes.

Example

class Employee:
    def __init__(self, name, id):
        self.id = id
        self.name = name

    def display(self):
        print("ID: %d \nName: %s" % (self.id, self.name))


emp1 = Employee("John", 101)
emp2 = Employee("David", 102)

# accessing display() method to print employee 1 information

emp1.display()

# accessing display() method to print employee 2 information
emp2.display()

Output:

ID: 101 
Name: John
ID: 102 
Name: David

Counting the number of objects of a class

The constructor is called automatically when we create the object of the class. Consider the following example.

Example

class Student:  
    count = 0  
    def __init__(self):  
        Student.count = Student.count + 1  
s1=Student()  
s2=Student()  
s3=Student()  
print("The number of students:",Student.count)

Output:

The number of students: 3

Python Non-Parameterized Constructor

The non-parameterized constructor uses when we do not want to manipulate the value or the constructor that has only self as an argument. Consider the following example.

Example

class Student:
    # Constructor - non parameterized
    def __init__(self):
        print("This is non parametrized constructor")
    def show(self,name):
        print("Hello",name)
student = Student()
student.show("John")

Python Parameterized Constructor

The parameterized constructor has multiple parameters along with the self. Consider the following example.

Example

class Student:
    # Constructor - parameterized
    def __init__(self, name):
        print("This is parametrized constructor")
        self.name = name
    def show(self):
        print("Hello",self.name)
student = Student("John")
student.show()

Output:

This is parametrized constructor
Hello John

Python Default Constructor

When we do not include the constructor in the class or forget to declare it, then that becomes the default constructor. It does not perform any task but initializes the objects. Consider the following example.

Example

class Student:
    roll_num = 101
    name = "Joseph"

    def display(self):
        print(self.roll_num,self.name)

st = Student()
st.display()

Output:

101 Joseph

More than One Constructor in Single class

Let's have a look at another scenario, what happen if we declare the two same constructors in the class.

Example

class Student:
    def __init__(self):
        print("The First Constructor")
    def __init__(self):
        print("The second contructor")

st = Student()

Output:

The Second Constructor

In the above code, the object st called the second constructor whereas both have the same configuration. The first method is not accessible by the st object. Internally, the object of the class will always call the last constructor if the class has multiple constructors.

Note: The constructor overloading is not allowed in Python.

Python built-in class functions

The built-in functions defined in the class are described in the following table.

SN	Function	Description
1	getattr(obj,name,default)	It is used to access the attribute of the object.
2	setattr(obj, name,value)	It is used to set a particular value to the specific attribute of an object.
3	delattr(obj, name)	It is used to delete a specific attribute.
4	hasattr(obj, name)	It returns true if the object contains some specific attribute.

Example

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

    # creates the object of the class Student
s = Student("John", 101, 22)

# prints the attribute name of the object s
print(getattr(s, 'name'))

# reset the value of attribute age to 23
setattr(s, "age", 23)

# prints the modified value of age
print(getattr(s, 'age'))

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

print(hasattr(s, 'id'))
# deletes the attribute age
delattr(s, 'age')

# this will give an error since the attribute age has been deleted
print(s.age)

Output:

John
23
True
AttributeError: 'Student' object has no attribute 'age'

Built-in class attributes

Along with the other attributes, a Python class also contains some built-in class attributes which provide information about the class.

The built-in class attributes are given in the below table.

SN	Attribute	Description
1	__dict__	It provides the dictionary containing the information about the class namespace.
2	__doc__	It contains a string which has the class documentation
3	__name__	It is used to access the class name.
4	__module__	It is used to access the module in which, this class is defined.
5	__bases__	It contains a tuple including all base classes.

Example

class Student:  
    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))  
s = Student("John",101,22)  
print(s.__doc__)  
print(s.__dict__)  
print(s.__module__)

Output:

None
{'name': 'John', 'id': 101, 'age': 22}
__main__

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Python Constructors

Python Constructor

Creating the constructor in python

Example

Counting the number of objects of a class

Example

Python Non-Parameterized Constructor

Example

Python Parameterized Constructor

Example

Python Default Constructor

Example

More than One Constructor in Single class

Example

Note: The constructor overloading is not allowed in Python.

Python built-in class functions

Example

Built-in class attributes

Example

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