def exempt_from_requirements(f):
    """
    Used to exempt a route handler from ambient requirement handling unless it
    is explicitly decorated with a requirement runner::

        @bp.route('/')
        @exempt_from_requirements
        def greeting_area():
            return "Hello!"

    To use with a class based view, apply it to the class level decorators
    attribute::

        class SomeCBV(View):
            decorators = [exempt_from_requirements]

            def get(self):
                return "Hello!"


    .. note::

        You cannot exempt individual methods of a class based view with this
        decorator, e.g. the follow will not work::

            class SomeCBV(MethodView):

                @exempt_from_requirements
                def get(self):
                    return "Hello!"


        Any permissioning applied at the blueprint level would still affect
        this route.


    :param f: The route handler to be decorated.

    .. versionadded:: 0.7.0
    """
    f.__allows_exempt__ = True
    return f
