code: def fetch(cert, use_deltas=True, user_agent=None, timeout=10):
      
  
      if not isinstance(cert, x509.Certificate):
          raise TypeError('cert must be an instance of asn1crypto.x509.Certificate, not %s' % type_name(cert))
  
      if user_agent is None:
          user_agent = 'certvalidator %s' % __version__
      elif not isinstance(user_agent, str_cls):
          raise TypeError('user_agent must be a unicode string, not %s' % type_name(user_agent))
  
      output = []
  
      sources = cert.crl_distribution_points
      if use_deltas:
          sources.extend(cert.delta_crl_distribution_points)
  
      for distribution_point in sources:
          url = distribution_point.url
          output.append(_grab_crl(user_agent, url, timeout))
  
      return output
docs: Fetches the CRLs for a certificate
  
      :param cert:
          An asn1cyrpto.x509.Certificate object to get the CRL for
  
      :param use_deltas:
          A boolean indicating if delta CRLs should be fetched
  
      :param user_agent:
          The HTTP user agent to use when requesting the CRL. If None,
          a default is used in the format "certvalidation 1.0.0".
  
      :param timeout:
          The number of seconds after which an HTTP request should timeout
  
      :raises:
          urllib.error.URLError/urllib2.URLError - when a URL/HTTP error occurs
          socket.error - when a socket error occurs
  
      :return:
          A list asn1crypto.crl.CertificateList objects