code: def _list_files(path, suffix=""):
      """
      Returns a list of all files ending in `suffix` contained within `path`.
  
      Parameters
      ----------
      path : str
          a filepath
      suffix : str
  
      Returns
      -------
      l : list
          A list of all files ending in `suffix` contained within `path`.
          (If `path` is a file rather than a directory, it is considered
          to "contain" itself)
      """
      if os.path.isdir(path):
          incomplete = os.listdir(path)
          complete = [os.path.join(path, entry) for entry in incomplete]
          lists = [_list_files(subpath, suffix) for subpath in complete]
          flattened = []
          for one_list in lists:
              for elem in one_list:
                  flattened.append(elem)
          return flattened
      else:
          assert os.path.exists(path), "couldn't find file '%s'" % path
          if path.endswith(suffix):
              return [path]
          return []
  
label_desc: Use image node_id 2 for calling a global function with example usage: _list_files(path, suffix) and returns: flattened, list, list