Fix LoadPolicy error handling + test.

Author: pp-mo

lib/iris/_combine.py

@@ -158,16 +158,23 @@ def set(self, options: str | dict | None = None, **kwargs):
         """
         if options is None:
             options_dict = {}
-        elif isinstance(options, str) and options in self.SETTINGS:
-            options_dict = self.SETTINGS[options]
+        elif isinstance(options, str):
+            if options in self.SETTINGS:
+                options_dict = self.SETTINGS[options]
+            else:
+                msg = (
+                    f"arg 'options'={options!r}, which is not a valid settings name, "
+                    f"expected one of {self.SETTINGS_NAMES}."
+                )
+                raise ValueError(msg)
         elif isinstance(options, dict):
             options_dict = options
         else:
-            msg = (
-                f"arg `options` has unexpected type {type(options)!r}, "
-                f"expected one of (None | str | dcit)."
-            )
-            raise TypeError(msg)
+            msg = (  # type: ignore[unreachable]
+                f"arg 'options' has unexpected type {type(options)!r}, "
+                f"expected one of (None | str | dict)."
+            )  # type: ignore[unreachable]
+            raise TypeError(msg)  # type: ignore[unreachable]
 
         # Override any options with keywords
         options_dict = options_dict.copy()  # don't modify original

lib/iris/tests/unit/test_LoadPolicy.py

@@ -36,7 +36,7 @@ def test_settings(self):
         options = LoadPolicy()
         settings = options.settings()
         assert isinstance(settings, dict)
-        assert tuple(settings.keys()) == LoadPolicy.OPTION_KEYS
+        assert list(settings.keys()) == LoadPolicy.OPTION_KEYS
         for key in LoadPolicy.OPTION_KEYS:
             assert settings[key] == getattr(options, key)
 
@@ -74,13 +74,16 @@ def test_arg_bad_dict(self):
 
     def test_arg_bad_string(self):
         options = LoadPolicy()
-        expected = "Invalid arg options='unknown' : must be a dict, or one of"
-        with pytest.raises(TypeError, match=expected):
-            options.set("unknown")
+        expected = (
+            r"arg 'options'='oddthing'.*not a valid setting.*expected one of.* "
+            "['legacy', 'default', 'recommended', 'comprehensive']"
+        )
+        with pytest.raises(ValueError, match=expected):
+            options.set("oddthing")
 
     def test_arg_bad_type(self):
         options = LoadPolicy()
-        expected = "must be a dict, or one of"
+        expected = r"arg 'options' has unexpected type \<class 'tuple'\>, expected one of \(None \| str \| dict\)\."
         with pytest.raises(TypeError, match=expected):
             options.set((1, 2, 3))