Callable with no fixed arguments

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In a Callable type, the first argument describes parameter types and the second describes the return type. An empty list Callable[[], str] means a callable that takes zero arguments - too restrictive. Callable[[Any], str] means a callable that takes exactly one argument of any type. The ellipsis ... is the special form that means 'any number of arguments of any types'. Callable[..., str] is the right choice when you only care about the return type and want to accept any callable shape. You will see this in callback registries, event systems, and decorator utilities where the decorator wraps a function without caring about its parameter list.