Add "-> None" to methods without other types (#961)

This is so that if MyPy is used in a charm, it type checks them. MyPy
by default doesn't type check methods that have no type annotations on
them.

ops/charm.py

@@ -110,7 +110,7 @@ class ActionEvent(EventBase):
     params: Dict[str, Any]
     """The parameters passed to the action."""
 
-    def defer(self):
+    def defer(self) -> None:
         """Action events are not deferrable like other events.
 
         This is because an action runs synchronously and the administrator
@@ -737,7 +737,7 @@ class SecretRotateEvent(SecretEvent):
     revision by calling :meth:`event.secret.set_content() <ops.Secret.set_content>`.
     """
 
-    def defer(self):
+    def defer(self) -> None:
         """Secret rotation events are not deferrable (Juju handles re-invocation)."""
         raise RuntimeError(
             'Cannot defer secret rotation events. Juju will keep firing this '
@@ -817,7 +817,7 @@ def restore(self, snapshot: Dict[str, Any]):
         super().restore(snapshot)
         self._revision = cast(int, snapshot['revision'])
 
-    def defer(self):
+    def defer(self) -> None:
         """Secret expiration events are not deferrable (Juju handles re-invocation)."""
         raise RuntimeError(
             'Cannot defer secret expiration events. Juju will keep firing '

ops/framework.py

@@ -148,7 +148,7 @@ def key(self) -> str:
         return self._key
 
     @property
-    def path(self):
+    def path(self) -> str:
         """Return the handle's path."""
         return self._path
 
@@ -194,7 +194,7 @@ def __init__(self, handle: Handle):
     def __repr__(self):
         return f"<{self.__class__.__name__} via {self.handle}>"
 
-    def defer(self):
+    def defer(self) -> None:
         """Defer the event to the future.
 
         Deferring an event from a handler puts that handler into a queue, to be
@@ -625,19 +625,20 @@ def __init__(self, storage: Union[SQLiteStorage, JujuStorage],
         else:
             self._juju_debug_at: Set[str] = set()
 
-    def set_breakpointhook(self):
-        """Hook into sys.breakpointhook so the builtin breakpoint() works as expected.
+    def set_breakpointhook(self) -> Optional[Any]:
+        """Hook into ``sys.breakpointhook`` so the builtin ``breakpoint()`` works as expected.
 
         This method is called by ``main``, and is not intended to be
         called by users of the framework itself outside of perhaps
         some testing scenarios.
 
-        It returns the old value of sys.excepthook.
-
-        The breakpoint function is a Python >= 3.7 feature.
+        The ``breakpoint()`` function is a Python >= 3.7 feature.
 
         This method was added in ops 1.0; before that, it was done as
-        part of the Framework's __init__.
+        part of the Framework's ``__init__``.
+
+        Returns:
+            The old value of ``sys.breakpointhook``.
         """
         old_breakpointhook = getattr(sys, 'breakpointhook', None)
         if old_breakpointhook is not None:
@@ -646,7 +647,7 @@ def set_breakpointhook(self):
             sys.breakpointhook = self.breakpoint
         return old_breakpointhook
 
-    def close(self):
+    def close(self) -> None:
         """Close the underlying backends."""
         self._storage.close()
 
@@ -664,7 +665,7 @@ def _forget(self, obj: 'Serializable'):
         """Stop tracking the given object. See also _track."""
         self._objects.pop(obj.handle.path, None)
 
-    def commit(self):
+    def commit(self) -> None:
         """Save changes to the underlying backends."""
         # Give a chance for objects to persist data they want to before a commit is made.
         self.on.pre_commit.emit()
@@ -831,7 +832,7 @@ def _emit(self, event: EventBase):
         if saved:
             self._reemit(event_path)
 
-    def reemit(self):
+    def reemit(self) -> None:
         """Reemit previously deferred events to the observers that deferred them.
 
         Only the specific observers that have previously deferred the event will be
@@ -974,7 +975,7 @@ def breakpoint(self, name: Optional[str] = None):
                 "Breakpoint %r skipped (not found in the requested breakpoints: %s)",
                 name, indicated_breakpoints)
 
-    def remove_unreferenced_events(self):
+    def remove_unreferenced_events(self) -> None:
         """Remove events from storage that are not referenced.
 
         In older versions of the framework, events that had no observers would get recorded but

ops/model.py

@@ -1234,7 +1234,7 @@ def remove_revision(self, revision: int):
             self._id = self.get_info().id
         self._backend.secret_remove(typing.cast(str, self.id), revision=revision)
 
-    def remove_all_revisions(self):
+    def remove_all_revisions(self) -> None:
         """Remove all revisions of this secret.
 
         This is called when the secret is no longer needed, for example when
@@ -1885,11 +1885,11 @@ def can_connect(self) -> bool:
             return False
         return True
 
-    def autostart(self):
+    def autostart(self) -> None:
         """Autostart all services marked as ``startup: enabled``."""
         self._pebble.autostart_services()
 
-    def replan(self):
+    def replan(self) -> None:
         """Replan all services: restart changed services and start startup-enabled services."""
         self._pebble.replan_services()
 

ops/storage.py

@@ -86,11 +86,11 @@ def _setup(self):
                 ''')
             self._db.commit()
 
-    def close(self):
+    def close(self) -> None:
         """Part of the Storage API, close the storage backend."""
         self._db.close()
 
-    def commit(self):
+    def commit(self) -> None:
         """Part of the Storage API, commit latest changes in the storage backend."""
         self._db.commit()
 
@@ -203,13 +203,13 @@ class JujuStorage:
     def __init__(self, backend: Optional['_JujuStorageBackend'] = None):
         self._backend: _JujuStorageBackend = backend or _JujuStorageBackend()
 
-    def close(self):
+    def close(self) -> None:
         """Part of the Storage API, close the storage backend.
 
         Nothing to be done for Juju backend, as it's transactional.
         """
 
-    def commit(self):
+    def commit(self) -> None:
         """Part of the Storage API, commit latest changes in the storage backend.
 
         Nothing to be done for Juju backend, as it's transactional.

ops/testing.py

@@ -1141,7 +1141,7 @@ def set_planned_units(self, num_units: int) -> None:
             raise TypeError("num_units must be 0 or a positive integer.")
         self._backend._planned_units = num_units
 
-    def reset_planned_units(self):
+    def reset_planned_units(self) -> None:
         """Reset the planned units override.
 
         This allows the harness to fall through to the built in methods that will try to