Throw error on old nbformat versions

convert.go

@@ -181,6 +181,12 @@ func ConvertString(writer io.Writer, notebookString string) error {
 		return err
 	}
 
+	if notebook.NBFormat < 4 {
+		return fmt.Errorf(
+			"the provided Jupyter Notebook uses an old version of the Notebook file format (version 4 or higher is required)",
+		)
+	}
+
 	// Get format extension of Jupyter Kernel language (e.g. "py")
 	fileExtension := notebook.Metadata.LanguageInfo.FileExtension[1:]
 

parse.go

@@ -54,9 +54,10 @@ type metadata struct {
 
 // notebook represents the JSON data structure in which a Jupyter Notebook is stored.
 type notebook struct {
-	Cells    []cell   `json:"cells"`
-	Metadata metadata `json:"metadata"`
-	// Omitted fields: "nbformat", "nbformat_minor"
+	Cells         []cell   `json:"cells"`
+	Metadata      metadata `json:"metadata"`
+	NBFormat      int      `json:"nbformat"`
+	NBFormatMinor int      `json:"nbformat_minor"`
 }
 
 // parseNotebook takes the provided Jupyter Notebook JSON string and parses it into the

test_data.go

@@ -395,5 +395,7 @@ var testParsedNotebook = notebook{
 		testCodeCell,
 		testRawCell,
 	},
-	Metadata: testMetadata,
+	Metadata:      testMetadata,
+	NBFormat:      4,
+	NBFormatMinor: 4,
 }