fix the last step checker logic

api/src/opentrons/protocol_api/core/engine/instrument.py

@@ -1043,9 +1043,19 @@ def _pick_up_tip() -> None:
                 air_gap=0,
             )
         ]
-        for step_volume, src_dest_combo in source_dest_per_volume_step:
+        next_step_volume, next_src_dest_combo = next(source_dest_per_volume_step)
+        is_last_step = False
+        while not is_last_step:
+            step_volume = next_step_volume
+            src_dest_combo = next_src_dest_combo
             step_source, step_destination = src_dest_combo
-            is_last_step = len([source_dest_per_volume_step]) > 0
+            try:
+                next_step_volume, next_src_dest_combo = next(
+                    source_dest_per_volume_step
+                )
+            except StopIteration:
+                is_last_step = True
+
             if new_tip == TransferTipPolicyV2.ALWAYS or (
                 new_tip == TransferTipPolicyV2.PER_SOURCE and step_source != prev_src
             ):

api/tests/opentrons/protocol_api_integration/test_transfer_with_liquid_classes.py

@@ -235,3 +235,129 @@ def test_order_of_water_transfer_steps(
         ]
         assert len(mock_manager.mock_calls) == 9
         assert mock_manager.mock_calls == expected_calls
+
+
+@pytest.mark.ot3_only
+@pytest.mark.parametrize(
+    "simulated_protocol_context", [("2.20", "Flex")], indirect=True
+)
+def test_order_of_water_transfer_steps_with_no_new_tips(
+    decoy: Decoy, mock_feature_flags: None, simulated_protocol_context: ProtocolContext
+) -> None:
+    """It should run the transfer steps without any errors.
+
+    This test only checks that various supported configurations for a transfer
+    analyze successfully. It doesn't check whether the steps are as expected.
+    That will be covered in analysis snapshot tests.
+    """
+    decoy.when(ff.allow_liquid_classes(RobotTypeEnum.FLEX)).then_return(True)
+    trash = simulated_protocol_context.load_trash_bin("A3")
+    tiprack = simulated_protocol_context.load_labware(
+        "opentrons_flex_96_tiprack_50ul", "D1"
+    )
+    pipette_50 = simulated_protocol_context.load_instrument(
+        "flex_1channel_50", mount="left", tip_racks=[tiprack]
+    )
+    nest_plate = simulated_protocol_context.load_labware(
+        "nest_96_wellplate_200ul_flat", "C3"
+    )
+    arma_plate = simulated_protocol_context.load_labware(
+        "armadillo_96_wellplate_200ul_pcr_full_skirt", "C2"
+    )
+
+    water = simulated_protocol_context.define_liquid_class("water")
+    pipette_50.pick_up_tip()
+    with (
+        mock.patch.object(
+            InstrumentCore,
+            "load_liquid_class",
+            side_effect=InstrumentCore.load_liquid_class,
+            autospec=True,
+        ) as patched_load_liquid_class,
+        mock.patch.object(
+            InstrumentCore,
+            "pick_up_tip",
+            side_effect=InstrumentCore.pick_up_tip,
+            autospec=True,
+        ) as patched_pick_up_tip,
+        mock.patch.object(
+            InstrumentCore,
+            "aspirate_liquid_class",
+            side_effect=InstrumentCore.aspirate_liquid_class,
+            autospec=True,
+        ) as patched_aspirate,
+        mock.patch.object(
+            InstrumentCore,
+            "dispense_liquid_class",
+            side_effect=InstrumentCore.dispense_liquid_class,
+            autospec=True,
+        ) as patched_dispense,
+        mock.patch.object(
+            InstrumentCore,
+            "drop_tip_in_disposal_location",
+            side_effect=InstrumentCore.drop_tip_in_disposal_location,
+            autospec=True,
+        ) as patched_drop_tip,
+    ):
+        mock_manager = mock.Mock()
+        mock_manager.attach_mock(patched_pick_up_tip, "pick_up_tip")
+        mock_manager.attach_mock(patched_load_liquid_class, "load_liquid_class")
+        mock_manager.attach_mock(patched_aspirate, "aspirate_liquid_class")
+        mock_manager.attach_mock(patched_dispense, "dispense_liquid_class")
+        mock_manager.attach_mock(patched_drop_tip, "drop_tip_in_disposal_location")
+        pipette_50.transfer_liquid(
+            liquid_class=water,
+            volume=40,
+            source=nest_plate.rows()[0][:2],
+            dest=arma_plate.rows()[0][:2],
+            new_tip="never",
+            trash_location=trash,
+        )
+        expected_calls = [
+            mock.call.load_liquid_class(
+                mock.ANY,
+                name="water",
+                transfer_properties=mock.ANY,
+                tiprack_uri="opentrons/opentrons_flex_96_tiprack_50ul/1",
+            ),
+            mock.call.aspirate_liquid_class(
+                mock.ANY,
+                volume=40,
+                source=mock.ANY,
+                transfer_properties=mock.ANY,
+                transfer_type=TransferType.ONE_TO_ONE,
+                tip_contents=[LiquidAndAirGapPair(liquid=0, air_gap=0)],
+            ),
+            mock.call.dispense_liquid_class(
+                mock.ANY,
+                volume=40,
+                dest=mock.ANY,
+                source=mock.ANY,
+                transfer_properties=mock.ANY,
+                transfer_type=TransferType.ONE_TO_ONE,
+                tip_contents=[LiquidAndAirGapPair(liquid=40, air_gap=0.1)],
+                add_final_air_gap=True,
+                trash_location=mock.ANY,
+            ),
+            mock.call.aspirate_liquid_class(
+                mock.ANY,
+                volume=40,
+                source=mock.ANY,
+                transfer_properties=mock.ANY,
+                transfer_type=TransferType.ONE_TO_ONE,
+                tip_contents=[LiquidAndAirGapPair(liquid=0, air_gap=0.1)],
+            ),
+            mock.call.dispense_liquid_class(
+                mock.ANY,
+                volume=40,
+                dest=mock.ANY,
+                source=mock.ANY,
+                transfer_properties=mock.ANY,
+                transfer_type=TransferType.ONE_TO_ONE,
+                tip_contents=[LiquidAndAirGapPair(liquid=40, air_gap=0.1)],
+                add_final_air_gap=False,
+                trash_location=mock.ANY,
+            ),
+        ]
+        assert len(mock_manager.mock_calls) == len(expected_calls)
+        assert mock_manager.mock_calls[2] == expected_calls[2]