Address review comments.

* Assume that the application will always be provided for relation events, even though there is an open Juju bug that indicates that may not always be the case at the moment.
* Minor style fixes.
* Don't adjust the types in the , as charmers may be relying on the default value in tests, and there's no valid default value other than None.

ops/charm.py

@@ -369,12 +369,12 @@ class RelationEvent(HookEvent):
     """
 
     relation: 'Relation'
-    """The relation involved in this event."""
+    """The relation involved in this event.
+    """
 
-    app: Optional[model.Application]
+    app: model.Application
     """The remote application that has triggered this event.
-
-    This may be ``None`` for relation-broken events."""
+    """
 
     unit: Optional[model.Unit]
     """The remote unit that has triggered this event.
@@ -393,7 +393,12 @@ def __init__(self, handle: 'Handle', relation: 'Relation',
                 f'cannot create RelationEvent with application {app} and unit {unit}')
 
         self.relation = relation
-        self.app = app
+        if app is None:
+            logger.warning("'app' expected but not received.")
+            # Do an explicit assignment here so that we can contain the type: ignore.
+            self.app = None  # type: ignore
+        else:
+            self.app = app
         self.unit = unit
 
     def snapshot(self) -> Dict[str, Any]:
@@ -428,7 +433,8 @@ def restore(self, snapshot: Dict[str, Any]):
         if app_name:
             self.app = self.framework.model.get_app(app_name)
         else:
-            self.app = None
+            logger.warning("'app_name' expected in snapshot but not found.")
+            self.app = None  # type: ignore
 
         unit_name = snapshot.get('unit_name')
         if unit_name:
@@ -444,15 +450,9 @@ class RelationCreatedEvent(RelationEvent):
     can occur before units for those applications have started. All existing
     relations should be established before start.
     """
-    app: model.Application
-    """The remote application that has triggered this event."""
-
     unit: None
-    """Always ``None``."""
-
-    def __init__(self, handle: 'Handle', relation: 'Relation',
-                 app: model.Application, unit: None = None):
-        super().__init__(handle, relation, app, unit)
+    """Always ``None``.
+    """
 
 
 class RelationJoinedEvent(RelationEvent):
@@ -466,15 +466,9 @@ class RelationJoinedEvent(RelationEvent):
     remote ``private-address`` setting, which is always available when
     the relation is created and is by convention not deleted.
     """
-    app: model.Application
-    """The remote application that has triggered this event."""
-
     unit: model.Unit
-    """The remote unit that has triggered this event."""
-
-    def __init__(self, handle: 'Handle', relation: 'Relation',
-                 app: model.Application, unit: model.Unit):
-        super().__init__(handle, relation, app, unit)
+    """The remote unit that has triggered this event.
+    """
 
 
 class RelationChangedEvent(RelationEvent):
@@ -495,8 +489,6 @@ class RelationChangedEvent(RelationEvent):
     The settings that may be queried, or set, are determined by the relation’s
     interface.
     """
-    app: model.Application
-    """The remote application that has triggered this event."""
 
 
 class RelationDepartedEvent(RelationEvent):
@@ -517,15 +509,13 @@ class RelationDepartedEvent(RelationEvent):
     Once all callback methods bound to this event have been run for such a
     relation, the unit agent will fire the :class:`RelationBrokenEvent`.
     """
-    app: model.Application
-    """The remote application that has triggered this event."""
-
     unit: model.Unit
-    """The remote unit that has triggered this event."""
+    """The remote unit that has triggered this event.
+    """
 
     def __init__(self, handle: 'Handle', relation: 'Relation',
-                 app: model.Application,
-                 unit: model.Unit,
+                 app: Optional[model.Application] = None,
+                 unit: Optional[model.Unit] = None,
                  departing_unit_name: Optional[str] = None):
         super().__init__(handle, relation, app=app, unit=unit)
 
@@ -578,11 +568,8 @@ class RelationBrokenEvent(RelationEvent):
     are currently known locally.
     """
     unit: None
-    """Always ``None``."""
-
-    def __init__(self, handle: 'Handle', relation: 'Relation',
-                 app: Optional[model.Application] = None, unit: None = None):
-        super().__init__(handle, relation, app, unit)
+    """Always ``None``.
+    """
 
 
 class StorageEvent(HookEvent):

test/test_charm.py

@@ -199,7 +199,7 @@ def on_any_relation(self, event: ops.RelationEvent):
         charm.on['pro1'].relation_departed.emit(rel, app, unit)
         charm.on['pro-2'].relation_departed.emit(rel, app, unit)
         charm.on['peer1'].relation_broken.emit(rel, app)
-        charm.on['peer-2'].relation_broken.emit(rel)
+        charm.on['peer-2'].relation_broken.emit(rel, app)
 
         self.assertEqual(charm.seen, [
             'RelationJoinedEvent',