<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def sign(self, key, network_id=None):
 """Sign this transaction with a private key. A potentially already existing signature would be overridden. """

        if network_id is None:
            rawhash = utils.sha3(rlp.encode(unsigned_tx_from_tx(self), UnsignedTransaction))
        else:
            assert 1 <= network_id < 2**63 - 18
            rlpdata = rlp.encode(rlp.infer_sedes(self).serialize(self)[
                                 :-3] + [network_id, b'', b''])
            rawhash = utils.sha3(rlpdata)

        key = normalize_key(key)

        v, r, s = ecsign(rawhash, key)
        if network_id is not None:
            v += 8 + network_id * 2

        ret = self.copy(
            v=v, r=r, s=s
        )
        ret._sender = utils.privtoaddr(key)
        return ret