code: def cache_rate(self, currency, date, rate):
          
          if not self.is_supported(defaults.INTERNAL_CURRENCY):
              logger.info('Tried to cache unsupported currency "{}". Ignoring.'.format(currency))
          else:
              cache.set(_cache_key(currency, date), str(rate), _cache_timeout(date))
docs: Cache a rate for future use