How to type hint a class decorator?

[bschubert (Brian Schubert)]

If you want type checkers to be able to recognize that the return type is both compatible with the passed class as well as with HasNewAttrib, what you’d need is an intersection type. This is unfortunately a well known missing feature in Python’s type system. I think the best you can do for now is a union.

You can find some more info on the status of intersection types in links contained in this comment:

How can I return an instance inheriting from a user type and a library type?

This is commonly called “intersection types”. Here’s a few previous discussions/proposals: The lack of intersection types also got mentioned in the recent 2024 Python Typing Survey.

For more inspiration, you can take a look at how @dataclass is type hinted in typeshed. Note that even though @dataclass adds an attribute (__dataclass_fields__), its type hints can’t express that:

• typeshed/stdlib/dataclasses.pyi at 346fef21370ff640d5f8496ae2986e873cad39f4 · python/typeshed · GitHub
• typeshed/stdlib/_typeshed/__init__.pyi at 346fef21370ff640d5f8496ae2986e873cad39f4 · python/typeshed · GitHub