Handle importstring pre/post save hooks

Try access the __name__ attribute on the function form of the hook, and
if it doesn't exist then use the string as the name.

Closes #753

jupyter_server/services/contents/manager.py

@@ -128,8 +128,12 @@ def _validate_pre_save_hook(self, proposal):
         if not callable(value):
             raise TraitError("pre_save_hook must be callable")
         if self.pre_save_hook is not None:
+            try:
+                existing_name = self.pre_save_hook.__name__
+            except AttributeError:
+                existing_name = self.pre_save_hook
             warnings.warn(
-                f"Overriding existing pre_save_hook ({self.pre_save_hook.__name__}) with a new one ({value.__name__}).",
+                f"Overriding existing pre_save_hook ({existing_name}) with a new one ({value.__name__}).",
                 stacklevel=2,
             )
         return value
@@ -163,8 +167,12 @@ def _validate_post_save_hook(self, proposal):
         if not callable(value):
             raise TraitError("post_save_hook must be callable")
         if self.post_save_hook is not None:
+            try:
+                existing_name = self.post_save_hook.__name__
+            except AttributeError:
+                existing_name = self.post_save_hook
             warnings.warn(
-                f"Overriding existing post_save_hook ({self.post_save_hook.__name__}) with a new one ({value.__name__}).",
+                f"Overriding existing post_save_hook ({existing_name}) with a new one ({value.__name__}).",
                 stacklevel=2,
             )
         return value

tests/services/contents/test_config.py

@@ -18,6 +18,30 @@ def test_config_did_something(jp_server_config, jp_serverapp):
     )
 
 
+def example_pre_save_hook():
+    pass
+
+
+def example_post_save_hook():
+    pass
+
+
+@pytest.mark.parametrize(
+    "jp_server_config",
+    [
+        {
+            "ContentsManager": {
+                "pre_save_hook": "tests.services.contents.test_config.example_pre_save_hook",
+                "post_save_hook": "tests.services.contents.test_config.example_post_save_hook",
+            },
+        }
+    ],
+)
+def test_pre_post_save_hook_config(jp_serverapp, jp_server_config):
+    assert jp_serverapp.contents_manager.pre_save_hook.__name__ == "example_pre_save_hook"
+    assert jp_serverapp.contents_manager.post_save_hook.__name__ == "example_post_save_hook"
+
+
 async def test_async_contents_manager(jp_configurable_serverapp):
     config = {"ContentsManager": {"checkpoints_class": AsyncCheckpoints}}
     argv = [