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

def validate(self, bigchain, current_transactions=[]):
        """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
        """
        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