Fills the result cache with 'num' more entries (or until the results
        iterator is exhausted).

def _fill_cache(self, num=None):
        """
        Fills the result cache with 'num' more entries (or until the results
        iterator is exhausted).
        """
        if self._iter:
            try:
                for i in range(num or ITER_CHUNK_SIZE):
                    self._result_cache.append(next(self._iter))
            except StopIteration:
                self._iter = None