feature_07_metaclass_registry.md

7. Metaclass Registry

Concept summary

Metaclasses let you customize class creation itself. They are advanced but powerful for plugin registration, declarative frameworks, and class-level enforcement.

What C# developers usually expect

In C#, this often requires reflection scans, attributes, or explicit startup registration. Python metaclasses can register subclasses automatically when classes are defined.

How this example works

RegistryMeta inherits from type and overrides __new__. Whenever a class using this metaclass is defined, __new__ runs. It stores each concrete plugin class in RegistryMeta.plugins, skipping the base Plugin. When SlackPlugin is declared, it is automatically added to the registry. main() prints the sorted registry keys.

Why this matters in production

This pattern supports zero-boilerplate plugin discovery for integrations, command handlers, or serialization adapters. Frameworks can auto-register implementations without separate wiring code.

Common mistakes

Using metaclasses too early when simple decorators or explicit registries are enough. Hiding too much magic, which can make debugging harder for teams unfamiliar with the pattern. Mixing multiple metaclasses unintentionally can cause class creation conflicts.

Source

• feature_07_metaclass_registry.py

Exact example from code file

class RegistryMeta(type):
    plugins: dict[str, type] = {}

    def __new__(mcls, name, bases, namespace):
        cls = super().__new__(mcls, name, bases, namespace)
        if name != "Plugin":
            mcls.plugins[name] = cls
        return cls


class Plugin(metaclass=RegistryMeta):
    pass


class SlackPlugin(Plugin):
    pass


def main() -> None:
    print(sorted(RegistryMeta.plugins))


if __name__ == "__main__":
    main()

Run

uv run src/csharp_to_python_learning/python_only_features/feature_07_metaclass_registry.py

Expected output

• ['SlackPlugin']