Reproduce and fix #792 (#793)

docs/CHANGELOG.md

@@ -1,7 +1,7 @@
 Jupytext ChangeLog
 ==================
 
-1.11.3 (2021-05-??)
+1.11.3 (2021-06-??)
 -------------------
 
 **Changed**
@@ -10,6 +10,7 @@ Jupytext ChangeLog
 **Fixed**
 - Dependencies of the JupyterLab extension have been upgraded to fix a security vulnerability ([#783](https://github.com/mwouts/jupytext/issues/783))
 - Variables assigned from a magic command are commented out in `py` scripts ([#781](https://github.com/mwouts/jupytext/issues/781))
+- Fixed a round-trip issue on notebooks that have None/null in their metadata ([#792](https://github.com/mwouts/jupytext/issues/792))
 
 
 1.11.2 (2021-05-02)

jupytext/header.py

@@ -143,7 +143,7 @@ def recursive_update(target, update):
     for key in update:
         value = update[key]
         if value is None:
-            del target[key]
+            target.pop(key, None)
         elif isinstance(value, dict):
             target[key] = recursive_update(target.get(key, {}), value)
         else:

tests/test_cli.py

@@ -1360,3 +1360,19 @@ def test_use_source_timestamp(tmpdir, cwd_tmpdir, python_notebook, capsys, forma
             cm.get("test.ipynb")
     else:
         cm.get("test.ipynb")
+
+
+def test_round_trip_with_null_metadata_792(tmpdir, cwd_tmpdir, python_notebook):
+    nb = python_notebook
+    nb.metadata.kernelspec = {
+        "argv": ["python", "-m", "ipykernel_launcher", "-f", "{connection_file}"],
+        "display_name": "Python 3",
+        "env": None,
+        "interrupt_mode": "signal",
+        "language": "python",
+        "metadata": None,
+        "name": "python3",
+    }
+    write(nb, "test.ipynb")
+    jupytext(["--to", "py:percent", "test.ipynb"])
+    jupytext(["--to", "ipynb", "test.py"])