code: def _AddFieldPaths(node, prefix, field_mask):
    
    if not node:
      field_mask.paths.append(prefix)
      return
    for name in sorted(node):
      if prefix:
        child_path = prefix + '.' + name
      else:
        child_path = name
      _AddFieldPaths(node[name], child_path, field_mask)
docs: Adds the field paths descended from node to field_mask.