Modifying multiple existent Ipython Notebooks using nbformat API

modify_notebooks.py

import os
import nbformat
warning_message = '### Developers warning: This notebook was generated using XPTO'

nb_warning_cell = nbformat.v4.new_markdown_cell(warning_message)
for root, dirs, files in os.walk("."):
    for file in files:
        if file[-6:] == '.ipynb':
            nb_path = os.path.join(root, file)
            current_nb = nbformat.read(nb_path, as_version=4)
            current_nb['cells'].insert(0, nb_warning_cell)
            nbformat.write(current_nb, nb_path)