Allow TaskMetadata to handle empty lists

python/lsst/pipe/base/_task_metadata.py

@@ -432,7 +432,12 @@ def __getitem__(self, key: str) -> Any:
             if key0 in self.metadata:
                 return self.metadata[key0]
             if key0 in self.arrays:
-                return self.arrays[key0][-1]
+                arr = self.arrays[key0]
+                if not arr:
+                    # If there are no elements then returning a scalar
+                    # is an error.
+                    raise KeyError(f"'{key}' not found")
+                return arr[-1]
             raise KeyError(f"'{key}' not found")
         # Hierarchical lookup so the top key can only be in the metadata
         # property. Trap KeyError and reraise so that the correct key
@@ -613,6 +618,8 @@ def _validate_value(self, value: Any) -> tuple[str, Any]:
             # For model consistency, need to check that every item in the
             # list has the same type.
             value = list(value)
+            if not value:
+                return "array", value
 
             type0 = type(value[0])
             for i in value:

tests/test_taskmetadata.py

@@ -91,6 +91,10 @@ def testTaskMetadata(self):
         self.assertEqual(meta.getArray("new_array"), ["a", "b", "c"])
         meta["new_array"] = [1, 2, 3]
         self.assertEqual(meta.getArray("new_array"), [1, 2, 3])
+        meta["empty_array"] = []
+        self.assertEqual(meta.getArray("empty_array"), [])
+        with self.assertRaises(KeyError):
+            meta["empty_array"]
 
         meta["meta"] = 5
         meta["meta"] = TaskMetadata()