fix: Allow null kernelspec in OverwriteKernelspec

The Notebook.kernelspec field is nullable, so in the event of a null
value, OverwriteKernelspec should not fail. This commit checks for that
case before trying to `json.loads` the value.

Author: shreve

nbgrader/preprocessors/overwritekernelspec.py

@@ -17,14 +17,14 @@ def preprocess(self, nb: NotebookNode, resources: ResourcesDict) -> Tuple[Notebo
         db_url = resources['nbgrader']['db_url']
 
         with Gradebook(db_url) as gb:
-            kernelspec = json.loads(
-                gb.find_notebook(notebook_id, assignment_id).kernelspec)
-            self.log.debug("Source notebook kernelspec: {}".format(kernelspec))
-            self.log.debug(
-                "Submitted notebook kernelspec: {}"
-                "".format(nb.metadata.get('kernelspec', None))
-            )
-            if kernelspec:
+            kernelspec = gb.find_notebook(notebook_id, assignment_id).kernelspec
+            if kernelspec is not None:
+                kernelspec = json.loads(kernelspec)
+                self.log.debug("Source notebook kernelspec: {}".format(kernelspec))
+                self.log.debug(
+                    "Submitted notebook kernelspec: {}"
+                    "".format(nb.metadata.get('kernelspec', None))
+                )
                 self.log.debug(
                     "Overwriting submitted notebook kernelspec: {}"
                     "".format(kernelspec)

nbgrader/tests/preprocessors/test_overwritekernelspec.py

@@ -41,7 +41,7 @@ class TestOverwriteKernelSpec(BaseTestPreprocessor):
 
     def test_overwrite_kernelspec(self, preprocessors, resources, gradebook):
         kernelspec = dict(
-            display_name='blarg',
+            display_name='Python 3',
             name='python3',
             language='python',
         )
@@ -57,3 +57,22 @@ def test_overwrite_kernelspec(self, preprocessors, resources, gradebook):
         notebook = gradebook.find_notebook("test", "ps0")
         assert nb.metadata['kernelspec'] == kernelspec
         assert json.loads(notebook.kernelspec) == kernelspec
+
+    def test_overwrite_kernelspec_null_spec(self, preprocessors, resources, gradebook):
+        # Create a notebook without a kernelspec
+        gradebook.add_notebook("test", "ps0")
+
+        kernelspec = dict(
+            display_name="Python 3",
+            name="python3",
+            language="python",
+        )
+
+        nb = new_notebook()
+        nb.metadata["kernelspec"] = kernelspec
+        nb, resources = preprocessors[1].preprocess(nb, resources)
+
+        validate(nb)
+        notebook = gradebook.find_notebook("test", "ps0")
+        assert notebook.kernelspec is None
+        assert nb.metadata["kernelspec"] == kernelspec