Class and instance attributes

In Python, you can group a class into data and create a new object type. Each instance can have attached attributes and can be used for content creation.

Class Attribute

An instance attribute is a Python variable belonging to one, and only one, object. This variable is only accessible in the scope of this object and it is defined inside the constructor function, __init__(self,..) of the class.

>>> class A:
...     a = "I am a class attribute!"
... 
>>> x = A()
>>> y = A()
>>> x.a
'I am a class attribute!'
>>> y.a
'I am a class attribute!'
>>> A.a
'I am a class attribute!'
>>>

Instance Attribute

A class attribute is a Python variable that belongs to a class rather than a particular object. It is shared between all the objects of this class and it is defined outside the constructor function, __init__(self,…), of the class.

# Python program to demonstrate
# instance attributes.
class emp:
      def __init__(self):
         self.name = 'xyz'
         self.salary = 4000
      def show(self):
         print self.name
         print self.salary
e1 = emp()
print "Dictionary form :", vars(e1)
print dir(e1)

Differences Between Class and Instance Attributes

Only one object is referred to in the case of a class attribute, while there may be multiple objects in the case of a set of instance attributes in the installation.

__dict__

A __dict__ is a dictionary or maping objet used to store an object’s attributes. How does Python deal with the object and class attributes using the __dict__ ? Well, each instance is stored in a dictonary.

The Pythonic way

class TestClass:
    instances = 0
    
    def __init__(self, name):
        TestClass.instances += 1
        self.name = name    
    
    @property
    def name(self):
        return self.__name    
    
    @name.setter
    def name(self, name):
        self.__name = name