code: def per_country_data(self):
          
          ret = {}
          for cache_date in self.cache_dates:
              data = self._cache_get(cache_date)
              ret[cache_date] = {}
              for cc, count in data['by_country'].items():
                  k = '%s (%s)' % (self._alpha2_to_country(cc), cc)
                  ret[cache_date][k] = count
              if len(ret[cache_date]) == 0:
                  ret[cache_date]['unknown'] = 0
          return ret
docs: Return download data by country.
  
          :return: dict of cache data; keys are datetime objects, values are
            dict of country (str) to count (int)
          :rtype: dict