Clear widgets metadata in clear (#49)

src/juv/_clear.py

@@ -10,16 +10,37 @@
 
 def clear(path: Path) -> None:
     nb = nbformat.read(path, nbformat.NO_CONVERT)
+    # clear cells
     for cell in nb.cells:
         if cell.cell_type == "code":
             cell.outputs = []
             cell.execution_count = None
+    # clear widgets metadata
+    nb.metadata.pop("widgets", None)
     nbformat.write(nb, path)
 
 
 def is_cleared(path: Path) -> bool:
-    """Check if a notebook has been cleared."""
+    """Check if a notebook has been cleared.
+
+    A notebook is considered cleared if:
+        - It does not have any outputs or execution counts in code cells.
+        - It does not have any widgets metadata.
+
+    Parameters
+    ----------
+    path : Path
+        Path to the notebook file.
+
+    Returns
+    -------
+    bool
+        True if the notebook is cleared, False otherwise
+
+    """
     nb = nbformat.read(path, nbformat.NO_CONVERT)
+    if "widgets" in nb.metadata:
+        return False
     for cell in filter(lambda cell: cell.cell_type == "code", nb.cells):
         if cell.outputs or cell.execution_count is not None:
             return False