docs: def default(self):
          """Return default contents"""
          import json
  
          import ambry.bundle.default_files as df
          import os
  
          path = os.path.join(os.path.dirname(df.__file__), 'notebook.ipynb')
  
          if six.PY2:
              with open(path, 'rb') as f:
                  content_str = f.read()
          else:
              # py3
              with open(path, 'rt', encoding='utf-8') as f:
                  content_str = f.read()
  
          c = json.loads(content_str)
  
          context = {
              'title': self._bundle.metadata.about.title,
              'summary': self._bundle.metadata.about.title,
              'bundle_vname': self._bundle.identity.vname
          }
  
          for cell in c['cells']:
              for i in range(len(cell['source'])):
                  cell['source'][i] = cell['source'][i].format(**context)
  
          c['metadata']['ambry'] = {
              'identity': self._bundle.identity.dict
          }
  
          return json.dumps(c, indent=4)