[Bug fix] Fail transaction builder when there is not enough input amount to pay for fees

Author: cffls

pycardano/txbuilder.py

@@ -161,6 +161,14 @@ def _calc_change(self, fees, inputs, outputs, address) -> List[TransactionOutput
         if self.mint:
             provided.multi_asset += self.mint
 
+        if not requested < provided:
+            raise InvalidTransactionException(
+                f"The input UTxOs cannot cover the transaction outputs and tx fee. \n"
+                f"Inputs: {inputs} \n"
+                f"Outputs: {outputs} \n"
+                f"fee: {fees}"
+            )
+
         change = provided - requested
 
         # Remove any asset that has 0 quantity

test/pycardano/test_txbuilder.py

@@ -198,3 +198,19 @@ def test_tx_builder_mint_multi_asset(chain_context):
     }
 
     assert expected == tx_body.to_primitive()
+
+
+def test_not_enough_input_amount(chain_context):
+    tx_builder = TransactionBuilder(chain_context)
+    sender = "addr_test1vrm9x2zsux7va6w892g38tvchnzahvcd9tykqf3ygnmwtaqyfg52x"
+    sender_address = Address.from_primitive(sender)
+    input_utxo = chain_context.utxos(sender)[0]
+
+    # Make output amount equal to the input amount
+    tx_builder.add_input(input_utxo).add_output(
+        TransactionOutput.from_primitive([sender, input_utxo.output.amount])
+    )
+
+    with pytest.raises(InvalidTransactionException):
+        # Tx builder must fail here because there is not enough amount of input ADA to pay tx fee
+        tx_body = tx_builder.build(change_address=sender_address)