Fix and test STAR setup discard of mounted tips

pylabrobot/liquid_handling/backends/hamilton/STAR_backend.py

@@ -2095,8 +2095,19 @@ async def set_up_pip():
         # Skip pip-channel I/O; the __init__ defaults stand in.
         # TODO: does not yet gate request_tip_presence or instrument-init moves.
         return
-      if not initialized or any(tip_presences):
+      has_tips = any(tip_presences)
+      if has_tips:
+        await self._initialize_pip_without_tip_discard()
+      elif not initialized:
         await self.initialize_pip()
+      if has_tips:
+        await self._discard_existing_tips_to_waste(tip_presences)
+        tip_presences_after_discard = await self.request_tip_presence()
+        if any(tip_presences_after_discard):
+          raise RuntimeError(
+            "Failed to discard tips during backend initialization: "
+            f"{tip_presences_after_discard}"
+          )
       self._channels_minimum_y_spacing = await self.channels_request_y_minimum_spacing()
 
       # VW is not supported on firmware from 2016 or older (see issue #1004). Skip the
@@ -6301,6 +6312,44 @@ async def initialize_pip(self):
       discarding_method=0,
     )
 
+  async def _initialize_pip_without_tip_discard(self) -> None:
+    """Initialize pip channels without using the DI command to discard mounted tips."""
+    dy = (4050 - 2175) // (self.num_channels - 1)
+    y_positions = [4050 - i * dy for i in range(self.num_channels)]
+
+    await self.initialize_pipetting_channels(
+      x_positions=[
+        int(self.extended_conf.tip_waste_x_position * 10)
+      ],  # Tip eject waste X position.
+      y_positions=y_positions,
+      begin_of_tip_deposit_process=int(self._channel_traversal_height * 10),
+      end_of_tip_deposit_process=1220,
+      z_position_at_end_of_a_command=3600,
+      tip_pattern=[False] * self.num_channels,
+      tip_type=4,  # TODO: get from tip types
+      discarding_method=0,
+    )
+
+  async def _discard_existing_tips_to_waste(self, tip_presences: List[Optional[bool]]) -> None:
+    """Discard tips found on channels during backend initialization."""
+    dy = (4050 - 2175) // (self.num_channels - 1)
+    y_positions = [4050 - i * dy for i in range(self.num_channels)]
+    x_position = int(self.extended_conf.tip_waste_x_position * 10)
+
+    trash = self.deck.get_trash_area()
+    max_z = trash.get_location_wrt(self.deck).z
+
+    await self.discard_tip(
+      x_positions=[x_position] * self.num_channels,
+      y_positions=y_positions,
+      tip_pattern=[bool(tip_present) for tip_present in tip_presences],
+      begin_tip_deposit_process=round((max_z + 59.9) * 10),
+      end_tip_deposit_process=round((max_z + 49.9) * 10),
+      minimum_traverse_height_at_beginning_of_a_command=int(self._channel_traversal_height * 10),
+      z_position_at_end_of_a_command=int(self._channel_traversal_height * 10),
+      discarding_method=TipDropMethod.PLACE_SHIFT,
+    )
+
   async def initialize_pipetting_channels(
     self,
     x_positions: List[int] = [0],

pylabrobot/liquid_handling/backends/hamilton/STAR_tests.py

@@ -2227,6 +2227,67 @@ async def test_1000uL_tips(self):
     tip_rack.unassign()
 
 
+class TestSTARSetupTipDiscard(unittest.IsolatedAsyncioTestCase):
+  """Tests for setup-time handling of tips already mounted on STAR pip channels."""
+
+  def _make_backend(self, num_channels: int = 4) -> STARBackend:
+    backend = STARBackend(read_timeout=1)
+    backend._num_channels = num_channels
+    backend._extended_conf = _DEFAULT_EXTENDED_CONFIGURATION
+    backend._iswap_parked = True
+    # The setup-time discard helper derives the TR Z range from the deck trash resource.
+    backend.set_deck(STARLetDeck())
+    backend._write_and_read_command = unittest.mock.AsyncMock(return_value="C0DIid0001er00/00")
+    return backend
+
+  async def test_initialize_pip_keeps_existing_active_tip_pattern(self):
+    backend = self._make_backend()
+
+    await backend.initialize_pip()
+
+    # The public initialization wrapper should keep its previous behavior for normal init.
+    backend._write_and_read_command.assert_has_calls(
+      [
+        _any_write_and_read_command_call(
+          "C0DIid0001xp08000&yp4050 3425 2800 2175tp2450tz1220te3600tm1 1 1 1tt04ti0"
+        )
+      ]
+    )
+
+  async def test_initialize_pip_without_tip_discard_uses_inactive_tip_pattern(self):
+    backend = self._make_backend()
+
+    await backend._initialize_pip_without_tip_discard()
+
+    # Mounted-tip setup uses DI for initialization only: tm0 prevents DI from discarding tips.
+    backend._write_and_read_command.assert_has_calls(
+      [
+        _any_write_and_read_command_call(
+          "C0DIid0001xp08000&yp4050 3425 2800 2175tp2450tz1220te3600tm0 0 0 0tt04ti0"
+        )
+      ]
+    )
+
+  async def test_discard_existing_tips_to_waste_uses_tr_with_tip_presence_pattern(self):
+    backend = self._make_backend()
+    backend._write_and_read_command.return_value = (
+      "C0TRid0001er00/00kz000 000 000 000vz000 000 000 000"
+    )
+
+    await backend._discard_existing_tips_to_waste([True, False, True, False])
+
+    # Actual setup-time tip disposal happens through TR and preserves the mounted-tip pattern.
+    # The tp/tz values come from the same trash-height formula used by normal drop_tips().
+    backend._write_and_read_command.assert_has_calls(
+      [
+        _any_write_and_read_command_call(
+          "C0TRid0001xp08000 08000 08000 08000yp4050 3425 2800 2175"
+          "tm1 0 1 0tp1970tz1870th2450te2450ti0"
+        )
+      ]
+    )
+
+
 class TestChannelsMinimumYSpacing(unittest.IsolatedAsyncioTestCase):
   """Test that different channel spacing configurations produce different behavior.