Old vs. New Style Classes in Python: What Are the Key Differences and When Should You Use Each?

Old Style vs. New Style Classes in Python

In Python, there are two distinct types of classes: old style and new style. This distinction has significant implications for class behaviour and type compatibility.

Old Style Classes

Old style classes were the only option prior to Python 2.2. They are characterized by:

• Separate class and type concepts: An old style instance's __class__ attribute points to its class, while type(x) always points to the built-in instance type.
• Limited inheritance: Old style classes can only inherit from other old style classes.

New Style Classes

New style classes were introduced in Python 2.2 to bridge the gap between classes and types. Their features include:

• Unified class and type: A new style instance's __class__ and type(x) usually refer to the same type.
• Enhanced inheritance: New style classes can inherit from built-in types and support diamond inheritance.
• Metamodel: New style classes provide a more robust meta-model, enabling features such as descriptors and metaclasses.

Key Differences

Apart from the above, old style and new style classes differ in:

• Method resolution order: Multiple inheritance in old style classes follows a depth-first approach, while new style classes use a breadth-first approach.
• Class object: Old style classes have a separate class object, while new style classes have a unique object-oriented class object with inherited methods.
• Instance.__class__: Old style instances return the exact class object, while new style instances may return a subclass that overrides __class__.

When to Use Old Style vs. New Style Classes

Python 2 supports both old style and new style classes, while Python 3 only supports new style classes. In Python 2, new style classes are recommended for their enhanced capabilities and compatibility across versions. However, old style classes may still be necessary for compatibility with legacy code.

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