'Construct a FetchResponse object from an OpenID library
SuccessResponse object.
@param success_response: A successful id_res response object
@type success_response: openid.consumer.consumer.SuccessResponse
@param signed: Whether non-signed args should be
processsed. If True (the default), only signed arguments
will be processsed.
@type signed: bool
@returns: A FetchResponse containing the data from the OpenID
message, or None if the SuccessResponse did not contain AX
extension data.
@raises AXError: when the AX data cannot be parsed.'

def fromSuccessResponse(cls, success_response, signed=True):

    self = cls()
    ax_args = success_response.extensionResponse(self.ns_uri, signed)
    try:
        self.parseExtensionArgs(ax_args)
    except NotAXMessage as err:
        return None
    else:
        return self
