Classes

Metaclasses

A metaclass customizes how classes themselves are created.

A metaclass customizes class creation. __new__ receives the class name, bases, and namespace before the class object exists.

Source

class Tagged(type):
    def __new__(mcls, name, bases, namespace):
        namespace["tag"] = name.lower()
        return super().__new__(mcls, name, bases, namespace)

print(Tagged.__name__)

Output

Tagged

The metaclass= keyword applies that class-building rule. Here the metaclass adds a tag attribute to the new class.

Source

class Event(metaclass=Tagged):
    pass

print(Event.tag)
print(type(Event).__name__)

Output

event
Tagged

Notes

Metaclasses customize class creation, not instance behavior directly.
Most code should prefer class decorators, functions, or ordinary inheritance.
You are most likely to meet metaclasses inside frameworks and ORMs.

Run the complete example

Expected output

event
Tagged

Execution time appears here after you run the example.

Metaclasses

See also

Run the complete example

Example code

Expected output