code: def list_downloadable_sources(target_dir):
      
      return [os.path.join(target_dir, fname)
              for fname in os.listdir(target_dir)
              if fname.endswith('.py')]
docs: Returns a list of python source files is target_dir
  
      Parameters
      ----------
      target_dir : str
          path to the directory where python source file are
      Returns
      -------
      list
          list of paths to all Python source files in `target_dir`