code: def ensure_file(url, path, md5sum=None):
      
  
      if not os.path.isfile(path) or (md5sum and md5sum != file_md5(path)):
          download_file(url, path)
docs: If file is not already at 'path', then download from 'url' and put it
      there.
  
      If md5sum is provided, and 'path' exists, check that file matches the
      md5sum.  If not, re-download.