Update the method for determining kernel language

The previous algorithm for `getNotebookType()` was based on getting the
kernel spec out of the current notebook. This is a problem when format
is triggered by, for example, `jupyterlab-autosave-on-focus-change`,
immediately after notebook creation. In that case, the language is
determined to be `null` and errors arise.

Now the `onSave()` handler waits for the `sessionContext` to be ready,
then `getNotebookType()` gets the same kernel spec info from the session
rather than the loaded notebook.

Author: shreve

src/formatter.ts

@@ -92,7 +92,7 @@ export class JupyterlabNotebookCodeFormatter extends JupyterlabCodeFormatter {
     return codeCells;
   }
 
-  private getNotebookType() {
+  private getNotebookType(): string | null {
     if (!this.notebookTracker.currentWidget) {
       return null;
     }
@@ -123,6 +123,16 @@ export class JupyterlabNotebookCodeFormatter extends JupyterlabCodeFormatter {
       }
     }
 
+    // finally, try to get the language from the current session's kernel spec
+    const sessionContext = this.notebookTracker.currentWidget?.sessionContext;
+    const kernelName = sessionContext?.session?.kernel?.name;
+    if (kernelName) {
+      const specs = sessionContext.specsManager.specs?.kernelspecs;
+      if (specs && kernelName in specs) {
+        return specs[kernelName]!.language;
+      }
+    }
+
     return null;
   }
 

src/index.ts

@@ -108,6 +108,7 @@ class JupyterLabCodeFormatter
     state: DocumentRegistry.SaveState
   ) {
     if (state === 'started' && this.config.formatOnSave) {
+      await context.sessionContext.ready;
       await this.notebookCodeFormatter.formatAllCodeCells(
         this.config,
         { saving: true },