What's the Difference Between Old and New Style Classes in Python?

Understanding the Distinction Between Old and New Style Classes in Python

In Python, the concept of old and new style classes plays a significant role in the object-oriented programming paradigm. Before diving into the key differences, it's essential to understand their historical context.

Old Style Classes: The Classic Approach

Up until Python 2.1, old style classes were the only option for developers. These classes were not directly connected to the concept of types. The type function would always return for any instance of an old style class, regardless of its actual class. This was due to the fact that all old style instances were implemented using a single built-in type called instance.

New Style Classes: A Unified Concept

In Python 2.2, new style classes were introduced to streamline the concepts of class and type. A new style class essentially represents a user-defined type. Instances of new style classes typically have type(x) return the same value as x.__class__, reflecting the unified object model.

Motivation Behind New Style Classes

Introducing new style classes had several compelling reasons:

• Unified Object Model: New style classes provided a cohesive object model with a comprehensive meta-model.
• Enhanced Capabilities: They enabled developers to subclass most built-in types and introduced descriptors, allowing for computed properties.

Creating New Style Classes

New style classes are created by leveraging another new style class or by specifying the "top-level type" object as the parent class in Python 2. In Python 3, all classes are considered new style by default.

Key Differences in Behavior

Besides the mentioned differences in type returns, new style classes offer a range of behavioral enhancements compared to old style classes. For instance, the invocation of special methods follows distinct rules, and the method resolution order in case of multi-inheritance has been improved.

Conclusion

The transition from old to new style classes in Python 2 and the exclusive use of new style classes in Python 3 marked a significant shift in object-oriented programming capabilities. Understanding the differences between these class types is crucial for effectively leveraging Python's object model and achieving optimal code design.

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