Python Metaclasses Descriptors

Metaclasses and Descriptors

Implementing Python metaclasses and descriptors — from new to descriptor protocol, init_subclass, and practical metaclass patterns.

When to Use

• Building DSLs and APIs with metaclasses
• Implementing property-like descriptors
• Automatic registration of subclasses
• ORM-like model definitions

Metaclass Patterns

class RegistryMeta(type):
    registry = {}
    def __new__(mcs, name, bases, namespace):
        cls = super().__new__(mcs, name, bases, namespace)
        if name != 'BaseModel':
            mcs.registry[name] = cls
        return cls

class BaseModel(metaclass=RegistryMeta): pass

class ValidatedField:
    """Descriptor pattern."""
    def __init__(self, validator): self.validator = validator
    def __set_name__(self, owner, name): self.name = name
    def __get__(self, obj, objtype=None): return obj.__dict__.get(self.name)
    def __set__(self, obj, value):
        self.validator(value)
        obj.__dict__[self.name] = value

Verification Checklist

• Metaclass new vs init understood
• Descriptor protocol (get, set, delete)
• set_name for automatic naming
• init_subclass as metaclass alternative (Python 3.6+)
• Practical use cases (registries, validation, ORMs)