code: def untar(original_tarball, output_directory):
      
      if not tarfile.is_tarfile(original_tarball):
          raise InvalidTarball
  
      tarball = tarfile.open(original_tarball)
      # set mtimes of members to now
      epochsecs = int(time())
      for member in tarball.getmembers():
          member.mtime = epochsecs
      tarball.extractall(output_directory)
  
      file_list = []
  
      for extracted_file in tarball.getnames():
          if extracted_file == '':
              break
          if extracted_file.startswith('./'):
              extracted_file = extracted_file[2:]
          # ensure we are actually looking at the right file
          extracted_file = os.path.join(output_directory, extracted_file)
  
          # Add to full list of extracted files
          file_list.append(extracted_file)
  
      return file_list
docs: Untar given tarball file into directory.
  
      Here we decide if our file is actually a tarball, then
      we untar it and return a list of extracted files.
  
      :param: tarball (string): the name of the tar file from arXiv
      :param: output_directory (string): the directory to untar in
  
      :return: list of absolute file paths