code: def validate(self, bigchain, current_transactions=[]):
          
          input_conditions = []
  
          if self.operation == Transaction.CREATE:
              duplicates = any(txn for txn in current_transactions if txn.id == self.id)
              if bigchain.is_committed(self.id) or duplicates:
                  raise DuplicateTransaction('transaction `{}` already exists'
                                             .format(self.id))
  
              if not self.inputs_valid(input_conditions):
                  raise InvalidSignature('Transaction signature is invalid.')
  
          elif self.operation == Transaction.TRANSFER:
              self.validate_transfer_inputs(bigchain, current_transactions)
  
          return self
docs: Validate transaction spend
          Args:
              bigchain (BigchainDB): an instantiated bigchaindb.BigchainDB object.
          Returns:
              The transaction (Transaction) if the transaction is valid else it
              raises an exception describing the reason why the transaction is
              invalid.
          Raises:
              ValidationError: If the transaction is invalid