Update documentation

docs/source/developers/savehooks.rst

@@ -16,6 +16,9 @@ They are both called with keyword arguments::
     pre_save_hook(model=model, path=path, contents_manager=cm)
     post_save_hook(model=model, os_path=os_path, contents_manager=cm)
 
+Assigning to ``pre_save_hook`` (or ``post_save_hook``) multiple times will
+register hooks, not override them. They will then be called sequentially.
+
 Examples
 --------
 

jupyter_server/services/contents/filemanager.py

@@ -96,7 +96,12 @@ def run_post_save_hook(self, model, os_path):
                 self.log.debug("Running post-save hook on %s", os_path)
                 post_save_hook(os_path=os_path, model=model, contents_manager=self)
             except Exception as e:
-                self.log.error("Post-save %s hook failed on %s", post_save_hook.__name__, os_path, exc_info=True)
+                self.log.error(
+                    "Post-save %s hook failed on %s",
+                    post_save_hook.__name__,
+                    os_path,
+                    exc_info=True,
+                )
                 raise web.HTTPError(
                     500, "Unexpected error while running post hook save: %s" % e
                 ) from e

jupyter_server/services/contents/manager.py

@@ -146,7 +146,12 @@ def run_pre_save_hook(self, model, path, **kwargs):
             except Exception:
                 # unhandled errors don't prevent saving,
                 # which could cause frustrating data loss
-                self.log.error("Pre-save hook %s failed on %s", pre_save_hook.__name__, path, exc_info=True)
+                self.log.error(
+                    "Pre-save hook %s failed on %s",
+                    pre_save_hook.__name__,
+                    path,
+                    exc_info=True,
+                )
 
     checkpoints_class = Type(Checkpoints, config=True)
     checkpoints = Instance(Checkpoints, config=True)