Python - Constructors

A constructor is an instance method in a class, that is automatically called whenever a new object of the class is created. The constructor's role is to assign value to instance variables as soon as the object is declared.

Python uses a special method called __init__() to initialize the instance variables for the object, as soon as it is declared.

Constructor Declaration

The __init__() method acts as a constructor. It needs a mandatory argument self, which is the reference to the object.

Syntax

The following is the syntax to declare a constructor in Python -

def __init__(self):
#initialize instance variables

The __init__() method as well as any instance method in a class has a mandatory parameter, self. However, you can give any name to the first parameter, not necessarily self.

Constructor Example

Let us define the constructor in the Employee class to initialize name and age as instance variables. We can then access these attributes of its object.

Python Constructor Implementation Code

class Employee:
   'Common base class for all employees'
   def __init__(self):
      self.name = "Bhavana"
      self.age = 24

e1 = Employee()
print ("Name: {}".format(e1.name))
print ("age: {}".format(e1.age))

It will produce the following output −

Name: Bhavana
age: 24

Parameterized Constructor

For the above Employee class, each object we declare will have same value for its instance variables name and age. To declare objects with varying attributes instead of the default, define arguments for the __init__() method. (A method is nothing but a function defined inside a class.)

Example

In this example, the __init__() constructor has two formal arguments. We declare Employee objects with different values −

class Employee:
   'Common base class for all employees'
   def __init__(self, name, age):
      self.name = name
      self.age = age

e1 = Employee("Bhavana", 24)
e2 = Employee("Bharat", 25)

print ("Name: {}".format(e1.name))
print ("age: {}".format(e1.age))
print ("Name: {}".format(e2.name))
print ("age: {}".format(e2.age))

It will produce the following output −

Name: Bhavana
age: 24
Name: Bharat
age: 25

You can assign defaults to the formal arguments in the constructor so that the object can be instantiated with or without passing parameters.

class Employee:
   'Common base class for all employees'
   def __init__(self, name="Bhavana", age=24):
      self.name = name
      self.age = age

e1 = Employee()
e2 = Employee("Bharat", 25)

print ("Name: {}".format(e1.name))
print ("age: {}".format(e1.age))
print ("Name: {}".format(e2.name))
print ("age: {}".format(e2.age))

It will produce the following output −

Name: Bhavana
age: 24
Name: Bharat
age: 25

Python - Instance Methods

In addition to the __init__() constructor, there may be one or more instance methods defined in a class. A method with self as one of the formal arguments is called instance method, as it is called by a specific object.

Example

In the following example a displayEmployee() method has been defined. It returns the name and age attributes of the Employee object that calls the method.

class Employee:
   def __init__(self, name="Bhavana", age=24):
      self.name = name
      self.age = age
   def displayEmployee(self):
      print ("Name : ", self.name, ", age: ", self.age)

e1 = Employee()
e2 = Employee("Bharat", 25)

e1.displayEmployee()
e2.displayEmployee()

It will produce the following output −

Name : Bhavana , age: 24
Name : Bharat , age: 25

You can add, remove, or modify attributes of classes and objects at any time −

emp1.salary = 7000 # Add a 'salary' attribute.
emp1.name = 'xyz' # Modify 'name' attribute.
del emp1.salary # Delete 'salary' attribute.

Instead of using the normal statements to access attributes, you can use the following functions −

  • The getattr(obj, name[, default]) − to access the attribute of object.

  • The hasattr(obj,name) − to check if an attribute exists or not.

  • The setattr(obj,name,value) − to set an attribute. If attribute does not exist, then it would be created.

  • The delattr(obj, name) − to delete an attribute.

print (hasattr(e1, 'salary')) # Returns true if 'salary' attribute exists
print (getattr(e1, 'name')) # Returns value of 'name' attribute
setattr(e1, 'salary', 7000) # Set attribute 'salary' at 8
delattr(e1, 'age') # Delete attribute 'age'

It will produce the following output −

False
Bhavana
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