(sorry if generic is the wrong word to describe this)

argparse.parse_args(namespace=MyNamespace()) returns an instance of MyNamespace, but it's annotated to always return an argparse.Namespace. This means that in order to have type-safety when accessing arguments by using a custom type-annotated class:

class Arguments:
    verbose: bool
    ...
args = argparse.parse_args(namespace=Arguments())

you currently need to cast the return value of argparse.parse_args.

Also, the passed object doesn't actually have to be a (subclass of) argparse.Namespace – a simple subclass of object is documented to work.