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Learn Python 3: Comprehensive Programming Guide

Metaclasses in Action

Explore how metaclasses in Python create classes and allow customization of the class creation process. Understand how overriding the __new__ method enables validation, automatic registration, and architectural control at the time classes are defined.

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Classes are often described as blueprints for creating objects. In Python, classes are also objects. Classes themselves are created by another construct. In Python, classes are created by metaclasses. Understanding this concept provides deeper insight into Python’s object system. In this lesson, we will examine how metaclasses enable declarative APIs that process class definitions during creation.

The class of the class

In Python, everything is an object. When we define a class, Python executes the class body and creates a new object representing that class. By default, all classes are instances of the built-in type.

We can verify this relationship using the type() function on a class itself.

Python
class SimpleData:
    pass
# Check the type of an instance
data = SimpleData()
print(f"Instance type: {type(data)}")
# Check the type of the class itself
print(f"Class type: {type(SimpleData)}")

  • Line 5: type(data) confirms that the object data was created by the class SimpleData.

  • Line 9: type(SimpleData) reveals that the class SimpleData was created by type. This means type is the default metaclass for all Python classes.

Because classes are just objects, the class keyword is ...