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Python › Metaclasses & Class Customization

What role does the class namespace dict play during class creation, and how can __prepare__ customize it?

Intermediate

Answer

During class body execution, Python populates a namespace mapping (by default a plain dict) with all the names defined in the class body (methods, class variables), which is then passed to the metaclass's __new__/__init__. A metaclass's __prepare__(mcs, name, bases) classmethod can return a custom mapping (like an OrderedDict or one that tracks definition order or rejects duplicate names) to control how that namespace behaves during class body execution.
class OrderedMeta(type):
    @classmethod
    def __prepare__(mcs, name, bases):
        return {}  # could return a custom ordered/tracking mapping

    def __new__(mcs, name, bases, namespace):
        cls = super().__new__(mcs, name, bases, namespace)
        cls._field_order = list(namespace.keys())
        return cls
Real-world example An ORM metaclass uses __prepare__ with a custom mapping to preserve and later use the exact declaration order of model fields for generating a consistent database schema.

Common follow-ups: Why is __prepare__ less necessary since Python 3.7 (dicts are ordered)?;What other customizations can __prepare__ enable?

Data Types & Structures;Descriptors & Properties
All 15 Metaclasses & Class Customization questions