Raise exception when attempting to deregister and un-registered toolkit

openforcefield/tests/test_toolkits.py

@@ -2407,11 +2407,15 @@ def test_deregister_toolkit_by_class(self):
         reason='RDKitToolkit and AmberToolsToolkit not available')
     def test_deregister_toolkit_bad_inputs(self):
         """Test bad inputs to deregister_toolkit"""
-        toolkit_registry = ToolkitRegistry(toolkit_precedence=[AmberToolsToolkitWrapper, RDKitToolkitWrapper])
+        toolkit_registry = ToolkitRegistry(toolkit_precedence=[AmberToolsToolkitWrapper])
 
         with pytest.raises(InvalidToolkitError):
             toolkit_registry.deregister_toolkit('rdkit as a string')
 
+        # Attempt to deregister a toolkit that is not registered
+        with pytest.raises(ToolkitUnavailableException):
+            toolkit_registry.deregister_toolkit(RDKitToolkitWrapper)
+
     def test_register_builtintoolkit(self):
         """Test creation of toolkit registry with Built-in toolkit"""
         # Test registration of BuiltInToolkitWrapper

openforcefield/utils/toolkits.py

@@ -4342,7 +4342,13 @@ def deregister_toolkit(self, toolkit_wrapper):
         for toolkit in self._toolkits:
             if type(toolkit) == type(toolkit_wrapper):
                 toolkits_to_remove.append(toolkit)
-        # TODO: add else clause to warn user if toolkits_to_remove is empty?
+
+        if not toolkits_to_remove:
+            msg = (
+                f"Did not find {toolkit_wrapper} in registry. "
+                f"Currently registered toolkits are {self._toolkits}"
+            )
+            raise ToolkitUnavailableException(msg)
 
         for toolkit_to_remove in toolkits_to_remove:
             self._toolkits.remove(toolkit_to_remove)