Return an exception if the namespace is not already installed

def install_required(f):
    """ Return an exception if the namespace is not already installed """

    @wraps(f)
    def wrapped(self, *args, **kwargs):
        if self.directory.new:
            raise SprinterException("Namespace %s is not yet installed!" % self.namespace)
        return f(self, *args, **kwargs)
    return wrapped