code: def list_files(d, extension=None):
      '''
      Lists files in a given directory.
      Args:
          d (str): Path to a directory.
          extension (str): If supplied, only files that contain the
              specificied extension will be returned. Default is ``False``,
              which returns all files in ``d``.
      Returns:
          list: A sorted list of file paths.
      '''
      if os.path.isdir(d):
          expanded_dir = os.path.expanduser(d)
          files = sorted(glob.glob(expanded_dir + '/*'))
      else:
          files = [d, ]
      if extension is not None:
          if type(extension) in STR_TYPES:
              extension = [extension, ]
          files = [f for f in files if any([f.split('.')[-1] in extension,
                                            f.split('.')[-1].upper() in extension,
                                            f.split('.')[-1].lower() in extension])]
      return files