Comparing Old-Style and New-Style Classes in Python

In Python, the difference between old-style and new-style classes is based on the inheritance from the built-in object class. This distinction was introduced in Python 2.x and was fully adopted in Python 3.x, where all classes are new-style classes. In this article, we will see the difference between the old-style and new-style classes in Python.

Python Old-Style Classes

In Python 2.x, old-style classes are those that do not explicitly inherit from the object class and thus have some limitations in terms of functionality and performance.

Syntax:

class OldStyleClass:
    pass

Following are the characteristics of the Python old-style classes:

• Do not support some advanced features such as descriptors, __slots__, and the new method resolution order (MRO).
• The MRO is implemented differently, using a depth-first search algorithm.

Python New-Style Classes

The New-style classes are those that explicitly inherit from the object class or any other built-in type that ultimately inherits from the object. It unify the concepts of types and classes, offering more features and a more consistent object-oriented model.

Syntax:

class NewStyleClass(object):
    pass

In Python 3.x, the object inheritance is implicit, so the following is also a new-style class:

class NewStyleClass:
    pass

The following are the characteristics of the Python new-style classes:

• Support advanced features such as descriptors, __slots__, properties, and the new method resolution order (C3 linearization).
• Provide a unified type hierarchy where all types derive from a common base class (object).
• More consistent and predictable behavior in multiple inheritance scenarios.

Differences between Old-Style Classes and New-Style Classes

Working on Old-Style and New-Style Classes in Python

Understanding the differences between old-style and new-style classes in Python is essential for working with different versions of Python and maintaining legacy code. Now let us see an example of both, old-style class and new-style on different Python versions for a better understanding.

Python 2.x

In Python 2.x, the type() function returns the type of the object. For old-style class instances, it returns <type 'instance'>. This is because old-style classes do not have the same rich type information as new-style classes. Whereas for new-style class, it returns the actual class of the instance in this case, <class '__main__.NewStyle'>. The __main__ indicates that this class is defined in the main module.

# Old-style class
class OldStyle:
    pass

# New-style class
class NewStyle(object):
    pass

# Instances
old_instance = OldStyle()
new_instance = NewStyle()

print(type(old_instance))  
print(type(new_instance)) 
print(isinstance(new_instance, object))

Output:

<type 'instance'>
<class '__main__.NewStyle'>
True

Python 3.x

In Python 3.x, all classes are new-style classes, and the distinction between old-style and new-style classes no longer exists. Thus, all classes in Python 3 automatically inherit from object, providing the benefits of new-style classes by default.

# Old-style class
class OldStyle:
    pass

# New-style class
class NewStyle(object):
    pass

# Instances
old_instance = OldStyle()
new_instance = NewStyle()

print(type(old_instance))  
print(type(new_instance)) 
print(isinstance(new_instance, object))

Output:

<class '__main__.OldStyle'>
<class '__main__.NewStyle'>
True

Conclusion

Old-style classes are a legacy concept from Python 2 that lacks many modern features and consistent behavior, while new-style classes, introduced in Python 2.x and used exclusively in Python 3.x, offer a unified and feature-rich class model. If you are working with Python 3, you do not need to worry about this distinction, as all classes are new-style by default.