Class and instance attributes

As an object-oriented language, Python provides two scopes for attributes: class attributes and instance attributes.

What’s a class attribute?

Class attributes are attributes which are owned by the class itself. They will be shared by all the instances of the class. Therefore they have the same value for every instance.So 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.

What’s an instance attribute?

Instance attributes are owned by the specific instances of a class. This means for two different instances the instance attributes are usually different.Therefore 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.

This is a basic Python class with two attributes: class attribute and instance attribute.

• height and width are an instances attribute defined inside the constructor.
• number of instances is a class attribute defined outside the constructor.

The differences between class and instance attributes

On the one hand theclass attributes is shared by all instances. When you change the value of a class attribute, it will affect all instances that share the same exact value. On the other hand instance attributes is unique to only that instance.

Advantages of instance attributes

• They are specific to an object and are easy to set and get thanks to properties.
• They are discarded once the instance is deleted, so they die with the instance they are associated with, which makes things clearer.

Advantages of class attributes

• All instances of the class inherit them from the class.
• They store data that is relevant to all the instances. For example, we could have a counter class attribute that increments every time we create a new instance and decrements every time we delete an instance.

Disadvantages of instance attributes

• They don’t allow keeping track of values in between instances.
• The fact that their values are lost on deletion is also a disadvantage in some cases where you want to keep a history of values for example.

Disadvantages of class attributes

• It can get messy when you create an instance in which the value of the class attribute is different than the class attribute value, and then try to retrieve it through another instance
• If you want to change a class attribute, you have to do it with the notation ClassName.AttributeName. Otherwise, you will create a new instance variable.

__dict__

Every object in Python has an attribute denoted by __dict__. This dictionary is an object contains all the attributes defined for the object itself. It maps the attribute name to its value.

The __dict__ of a class however, is not that straight-forward. Its actually an object of a class called dictproxy. dictproxy is a special class whose objects behave like normal dicts, but they differ in some key behaviours.

For instance attribute

For class attribute

Conclusion

Python class attributes may be useful in different cases, however, they must be used with caution in order to avoid unexpected behaviors.

PS:

Always remember that everything in Python is an object.