fix(workflows): resolve negative list indices in expressions

src/specify_cli/workflows/expressions.py

@@ -139,28 +139,29 @@ def _filter_from_json(value: Any) -> Any:
 # against it, and the condition gate below reuses it rather than describing the
 # same shape a second time, so widening what indexing accepts cannot leave the
 # evaluator and the gate disagreeing.
-_INDEXED_SEGMENT = re.compile(r"^([\w-]+)\[(\d+)\]$")
+_INDEXED_SEGMENT = re.compile(r"^([\w-]+)\[(-?\d+)\]$")
 
 _PLAIN_SEGMENT = re.compile(r"^[\w-]+$")
 
 
 def _resolve_dot_path(obj: Any, path: str) -> Any:
     """Resolve a dotted path like ``steps.specify.output.file`` against *obj*.
 
-    Supports dict key access and list indexing (e.g., ``task_list[0]``).
+    Supports dict key access and list indexing, including the negative form
+    Python and Jinja2 both accept (e.g., ``task_list[0]``, ``task_list[-1]``).
     """
     parts = path.split(".")
     current = obj
     for part in parts:
-        # Handle list indexing: name[0]
+        # Handle list indexing: name[0], name[-1]
         idx_match = _INDEXED_SEGMENT.match(part)
         if idx_match:
             key, idx = idx_match.group(1), int(idx_match.group(2))
             if isinstance(current, dict):
                 current = current.get(key)
             else:
                 return None
-            if isinstance(current, list) and 0 <= idx < len(current):
+            if isinstance(current, list) and -len(current) <= idx < len(current):
                 current = current[idx]
             else:
                 return None

tests/test_workflows.py

@@ -1013,6 +1013,25 @@ def test_list_indexing(self):
         result = evaluate_expression("{{ steps.tasks.output.task_list[0].file }}", ctx)
         assert result == "a.md"
 
+    def test_negative_list_indexing(self):
+        """``list[-1]`` resolves from the end, as Python and Jinja2 both do.
+
+        Without it the index silently fell through to a dict lookup for the
+        literal key ``"task_list[-1]"`` and produced ``None``, so a template
+        reaching for the last element rendered empty with no error.
+        """
+        from specify_cli.workflows.expressions import evaluate_expression
+        from specify_cli.workflows.base import StepContext
+
+        ctx = StepContext(
+            steps={"tasks": {"output": {"task_list": [{"file": "a.md"}, {"file": "b.md"}]}}}
+        )
+        assert evaluate_expression("{{ steps.tasks.output.task_list[-1].file }}", ctx) == "b.md"
+        assert evaluate_expression("{{ steps.tasks.output.task_list[-2].file }}", ctx) == "a.md"
+        # Out of range in either direction stays None rather than raising.
+        assert evaluate_expression("{{ steps.tasks.output.task_list[-3] }}", ctx) is None
+        assert evaluate_expression("{{ steps.tasks.output.task_list[2] }}", ctx) is None
+
     def test_context_run_id_resolves(self):
         """``{{ context.run_id }}`` resolves to ``StepContext.run_id``.
 

tests/unit/test_condition_expression_block.py

@@ -946,16 +946,17 @@ def test_a_literal_never_reaches_the_resolver():
 def test_gate_reads_the_shared_indexed_segment_definition(monkeypatch):
     """Widening `_INDEXED_SEGMENT` alone must reach the gate.
 
-    `steps.…​.task_list[-1]` is rejected today because `_INDEXED_SEGMENT` — the
-    one place `_resolve_dot_path` says what an index looks like — accepts digits
-    only. Widening it there and nowhere else must be enough; if the gate keeps
-    its own copy of the shape (as `_PATH_SEGMENT` used to), this fails.
+    `steps.…​.task_list[+1]` is rejected today because `_INDEXED_SEGMENT` — the
+    one place `_resolve_dot_path` says what an index looks like — accepts an
+    optional minus sign and digits only. Widening it there and nowhere else must
+    be enough; if the gate keeps its own copy of the shape (as `_PATH_SEGMENT`
+    used to), this fails.
     """
-    path = "steps.tasks.output.task_list[-1].file"
+    path = "steps.tasks.output.task_list[+1].file"
     assert expressions._unresolvable_term(path) is not None
 
     monkeypatch.setattr(
-        expressions, "_INDEXED_SEGMENT", re.compile(r"^([\w-]+)\[(-?\d+)\]$")
+        expressions, "_INDEXED_SEGMENT", re.compile(r"^([\w-]+)\[([+-]?\d+)\]$")
     )
     assert expressions._unresolvable_term(path) is None