def METHOD_NAME(cls, name, sign):
    """Extend client signature methods.
    :param name: A string to represent signature method.
    :param sign: A function to generate signature.
    The ``sign`` method accept 2 parameters::
        def custom_sign_method(client, request):
            # client is the instance of Client.
            return 'your-signed-string'
        Client.register_signature_method('custom-name', custom_sign_method)
    """
    cls.SIGNATURE_METHODS[name] = sign