Receive string data(byte array) from the server.

        return value: string(byte array) value.

def recv(self):
        """
        Receive string data(byte array) from the server.

        return value: string(byte array) value.
        """
        with self.readlock:
            opcode, data = self.recv_data()
        if six.PY3 and opcode == ABNF.OPCODE_TEXT:
            return data.decode("utf-8")
        elif opcode == ABNF.OPCODE_TEXT or opcode == ABNF.OPCODE_BINARY:
            return data
        else:
            return ''