Get the base64 encoded XDR string representing this
        :class:`TransactionEnvelope`.

def xdr(self):
        """Get the base64 encoded XDR string representing this
        :class:`TransactionEnvelope`.

        """
        te = Xdr.StellarXDRPacker()
        te.pack_TransactionEnvelope(self.to_xdr_object())
        te = base64.b64encode(te.get_buffer())
        return te