code: def sign(self):
          
          self.constructTx()
  
          if "operations" not in self or not self["operations"]:
              return
  
          # Legacy compatibility!
          # If we are doing a proposal, obtain the account from the proposer_id
          if self.blockchain.proposer:
              proposer = self.account_class(
                  self.blockchain.proposer, blockchain_instance=self.blockchain
              )
              self.wifs = set()
              self.signing_accounts = list()
              self.appendSigner(proposer["id"], "active")
  
          # We need to set the default prefix, otherwise pubkeys are
          # presented wrongly!
          if self.blockchain.rpc:
              self.operations.default_prefix = self.blockchain.rpc.chain_params["prefix"]
          elif "blockchain" in self:
              self.operations.default_prefix = self["blockchain"]["prefix"]
  
          try:
              signedtx = self.signed_transaction_class(**self.json())
          except Exception:
              raise ValueError("Invalid TransactionBuilder Format")
  
          if not any(self.wifs):
              raise MissingKeyError
  
          signedtx.sign(self.wifs, chain=self.blockchain.rpc.chain_params)
          self["signatures"].extend(signedtx.json().get("signatures"))
          return signedtx
docs: Sign a provided transaction with the provided key(s)
  
              :param dict tx: The transaction to be signed and returned
              :param string wifs: One or many wif keys to use for signing
                  a transaction. If not present, the keys will be loaded
                  from the wallet as defined in "missing_signatures" key
                  of the transactions.