Writing Custom Metaclasses (__new__ & __init__)

1Concept

A metaclass inherits from `type`. Overriding `__new__` allows inspecting, validating, or modifying class attributes and method signatures before the class is constructed in memory.

2Architecture Diagram

class Model(metaclass=ModelMeta): ---> ModelMeta.__new__() intercepts and validates fields

3Code Example

class EnforceUpperFieldsMeta(type):
    def __new__(mcls, name, bases, namespace):
        # Validate that all public methods have docstrings
        for attr_name, attr_val in namespace.items():
            if callable(attr_val) and not attr_name.startswith("_"):
                if not attr_val.__doc__:
                    print(f"[Warning] Method {attr_name}() in {name} lacks docstring!")
        return super().__new__(mcls, name, bases, namespace)

class PaymentGateway(metaclass=EnforceUpperFieldsMeta):
    def charge(self, amount: float):
        pass # Missing docstring!

print(f"Constructed class: {PaymentGateway.__name__}")

4Expected Output

[Warning] Method charge() in PaymentGateway lacks docstring!
Constructed class: PaymentGateway

5Key Takeaways

• ✓Metaclasses intercept class construction, NOT instance instantiation.
• ✓Always delegate to `super().__new__(mcls, name, bases, namespace)`.
• ✓Use `__init_subclass__` where possible as a simpler alternative to metaclasses.