code: def solve(self, hash160_lookup, tx_in_idx, hash_type=None, **kwargs):
          
          if hash_type is None:
              hash_type = SIGHASH_ALL
          kwargs["hash160_lookup"] = hash160_lookup
          if "signature_placeholder" not in kwargs:
              kwargs["signature_placeholder"] = generate_default_placeholder_signature(kwargs.get("generator"))
          if self.tx.txs_in[tx_in_idx].witness:
              kwargs["existing_script"] = self.tx.txs_in[tx_in_idx].witness
          else:
              kwargs["existing_script"] = [
                  data for opcode, data, pc, new_pc in self.ScriptTools.get_opcodes(
                      self.tx.txs_in[tx_in_idx].script) if data is not None]
          kwargs["signature_type"] = hash_type
          kwargs["generator_for_signature_type_f"] = self.solution_checker.VM.generator_for_signature_type
          constraints = self.determine_constraints(tx_in_idx, p2sh_lookup=kwargs.get("p2sh_lookup"))
          solution_list, witness_list = self.solve_for_constraints(constraints, **kwargs)
          solution_script = self.ScriptTools.compile_push_data_list(solution_list)
          if witness_list:
              return solution_script, witness_list
          return solution_script
docs: Sign a standard transaction.
          hash160_lookup:
              An object with a get method that accepts a hash160 and returns the
              corresponding (secret exponent, public_pair, is_compressed) tuple or
              None if it's unknown (in which case the script will obviously not be signed).
              A standard dictionary will do nicely here.
          tx_in_idx:
              the index of the tx_in we are currently signing