Let MessageException be imported from utils.toolkits with warning

openff/toolkit/utils/toolkits.py

@@ -27,7 +27,6 @@
     "ALLOWED_FRACTIONAL_BOND_ORDER_MODELS",
     "DEFAULT_CHARGE_MODEL",
     "ALLOWED_CHARGE_MODELS",
-    "MessageException",
     "IncompatibleUnitError",
     "MissingDependencyError",
     "MissingPackageError",
@@ -59,6 +58,7 @@
 
 
 import logging
+import warnings
 
 from openff.toolkit.utils.ambertools_wrapper import AmberToolsToolkitWrapper
 from openff.toolkit.utils.base_wrapper import ToolkitWrapper
@@ -96,6 +96,19 @@
 from openff.toolkit.utils.rdkit_wrapper import RDKitToolkitWrapper
 from openff.toolkit.utils.toolkit_registry import ToolkitRegistry
 
+deprecated_names = ["MessageException"]
+
+# TODO: Remove in January 2022, see _DeprecatedMessageException in openff/toolkit/utils/exceptions.py
+def __getattr__(name):
+    if name in deprecated_names:
+        warnings.filterwarnings("default", category=DeprecationWarning)
+        from openff.toolkit.utils.exceptions import MessageException
+
+        return MessageException
+        # return globals()[f"MessageException"]
+    raise AttributeError(f"module {__name__} has no attribute {name}")
+
+
 # =============================================================================================
 # CONFIGURE LOGGER
 # =============================================================================================