Add warnings to function, and test them

Author: Will Benfold

lib/iris/fileformats/netcdf.py

@@ -238,8 +238,11 @@ def _split_cell_methods(nc_cell_methods: str) -> List[re.Match]:
         elif cha == ")":
             bracket_depth -= 1
             if bracket_depth < 0:
-                pass
-                # TODO: Raise meaningful warning or error to indicate badly formatted cell method
+                msg = (
+                    "Cell methods may be incorrectly parsed due to mismatched "
+                    "brackets"
+                )
+                warnings.warn(msg, UserWarning, stacklevel=2)
         if bracket_depth > 0 and ind in name_start_inds:
             name_start_inds.remove(ind)
 
@@ -255,8 +258,11 @@ def _split_cell_methods(nc_cell_methods: str) -> List[re.Match]:
         nc_cell_method_str = nc_cell_methods[start_ind:end_ind]
         nc_cell_method_match = _CM_PARSE.match(nc_cell_method_str.strip())
         if not nc_cell_method_match:
-            pass
-            # TODO: Raise meaningful warning or error to indicate badly formatted cell method
+            msg = (
+                f"Failed to fully parse cell method string: {nc_cell_methods}"
+            )
+            warnings.warn(msg, UserWarning, stacklevel=2)
+            continue
         nc_cell_methods_matches.append(nc_cell_method_match)
 
     return nc_cell_methods_matches

lib/iris/tests/unit/fileformats/netcdf/test_parse_cell_methods.py

@@ -116,6 +116,36 @@ def test_comment_brackets(self):
             res = parse_cell_methods(cell_method_str)
             self.assertEqual(res, expected)
 
+    def test_comment_bracket_mismatch_warning(self):
+        cell_method_strings = [
+            "time: minimum within days (comment: 18h day-1)-18h)",
+            "time : minimum within days (comment: 18h day-1)-18h)",
+        ]
+        for cell_method_str in cell_method_strings:
+            with mock.patch("warnings.warn") as warn:
+                _ = parse_cell_methods(cell_method_str)
+                self.assertIn(
+                    "Cell methods may be incorrectly parsed due to mismatched brackets",
+                    warn.call_args[0][0],
+                )
+
+    def test_badly_formatted_warning(self):
+        cell_method_strings = [
+            # "time: maximum (interval: 1 hr comment: first bit "
+            # "time: mean (interval: 1 day comment: second bit)",
+            "time: (interval: 1 hr comment: first bit) "
+            "time: mean (interval: 1 day comment: second bit)",
+            "time: maximum (interval: 1 hr comment: first bit) "
+            "time: (interval: 1 day comment: second bit)",
+        ]
+        for cell_method_str in cell_method_strings:
+            with mock.patch("warnings.warn") as warn:
+                _ = parse_cell_methods(cell_method_str)
+                self.assertIn(
+                    f"Failed to fully parse cell method string: {cell_method_str}",
+                    warn.call_args[0][0],
+                )
+
     def test_portions_of_cells(self):
         cell_method_strings = [
             "area: mean where sea_ice over sea",