Mark function as external.

    :param func:
    :return:

def external(func):
    '''
    Mark function as external.

    :param func:
    :return:
    '''

    def f(*args, **kwargs):
        '''
        Stub.

        :param args:
        :param kwargs:
        :return:
        '''
        return func(*args, **kwargs)

    f.external = True
    f.__doc__ = func.__doc__

    return f