code: def _list_files(path, suffix=""):
    
    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 []
docs: 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)