fix(hamilton): resolve trash_core96 from the deck instead of a stale pointer

pylabrobot/resources/hamilton/hamilton_deck_tests.py

@@ -1,7 +1,7 @@
 import textwrap
 import unittest
 
-from pylabrobot.resources import TipRack
+from pylabrobot.resources import Deck, TipRack
 from pylabrobot.resources.corning import (
   cor_96_wellplate_360uL_Fb,
 )
@@ -94,6 +94,30 @@ def test_teaching_rack_excluded_by_nominal_volume_search(self):
     self.assertEqual(len(matches), 5)
     self.assertNotIn("teaching_tip_rack", {tr.name for tr in matches})
 
+  def test_get_trash_area96_survives_serialization_round_trip(self):
+    """`serialize()` encodes the 96 trash as a child and sets `with_trash96=False`, so the
+    rebuilt deck's `__init__` leaves the cached `_trash96` pointer as None. `get_trash_area96()`
+    must still resolve it from the child tree rather than raising."""
+    deck = self.build_layout()
+    original_location = deck.get_trash_area96().get_location_wrt(deck)
+
+    deck2 = Deck.deserialize(deck.serialize())
+
+    self.assertIn("trash_core96", [child.name for child in deck2.children])
+    self.assertEqual(deck2.get_trash_area96().get_location_wrt(deck2), original_location)
+
+  def test_get_trash_area96_raises_after_clear_include_trash(self):
+    """`clear(include_trash=True)` unassigns the 96 trash. `get_trash_area96()` must raise
+    rather than hand back the now-orphaned resource via the stale cached pointer."""
+    deck = self.build_layout()
+    deck.get_trash_area96()  # populate the cache before invalidating it
+
+    deck.clear(include_trash=True)
+
+    self.assertNotIn("trash_core96", [child.name for child in deck.children])
+    with self.assertRaises(RuntimeError):
+      deck.get_trash_area96()
+
   def test_assign_gigantic_resource(self):
     stanley_cup = StanleyCup_QUENCHER_FLOWSTATE_TUMBLER(name="HUGE")
     deck = STARLetDeck()

pylabrobot/resources/hamilton/hamilton_decks.py

@@ -548,11 +548,20 @@ def rails_to_location(self, rails: int) -> Coordinate:
     return Coordinate(x=x, y=63, z=100)
 
   def get_trash_area96(self) -> Trash:
-    if self._trash96 is None:
-      raise RuntimeError(
-        "Trash area for 96-well plates was not created. Initialize with `with_trash96=True`."
-      )
-    return self._trash96
+    # `_trash96` is only set in `__init__`, so it is stale whenever the deck was rebuilt from a
+    # serialized layout (which encodes the trash as a child and passes `with_trash96=False`) or the
+    # trash was unassigned. Resolve against the child tree instead, and only trust the cached
+    # reference while it is still assigned to this deck.
+    if self._trash96 is not None and self._trash96.parent is self:
+      return self._trash96
+
+    if self.has_resource("trash_core96"):
+      self._trash96 = cast(Trash, self.get_resource("trash_core96"))
+      return self._trash96
+
+    raise RuntimeError(
+      "Trash area for 96-well plates was not created. Initialize with `with_trash96=True`."
+    )
 
   def clear(self, include_trash: bool = False):
     """Clear the deck, removing all resources except the trash areas and the waste block."""