Optional output in sync_block (python)

Hello Everyone,

I have an LDPC decoder derived from sync_block in python. This is how
define the input and output ports.

    gr.sync_block.__init__(self, name="LDPC Decoder",
            in_sig = [(np.float32, self.N)],
            out_sig = [(np.int32, self.K)])

For BER analysis I would like to have another optional output (a vector
length self.N, (np.int32, self.N) ). How can I specify this optional