code: def maybe_download(self, filename, work_directory, source_url):
          
          if not os.path.exists(work_directory):
              os.makedirs(work_directory)
          filepath = os.path.join(work_directory, filename)
          if not os.path.exists(filepath):
              temp_file_name, _ = urllib.request.urlretrieve(source_url)
              copyfile(temp_file_name, filepath)
              print('Successfully downloaded', filename)
          return filepath
docs: Download the data from source url, unless it's already here.
          Args:
              filename: string, name of the file in the directory.
              work_directory: string, path to working directory.
              source_url: url to download from if file doesn't exist.
          Returns:
              Path to resulting file.