code: def needs_update(self, cache_key):
      
      if not self.cacheable(cache_key):
        # An uncacheable CacheKey is always out of date.
        return True
  
      return self._read_sha(cache_key) != cache_key.hash
docs: Check if the given cached item is invalid.
  
      :param cache_key: A CacheKey object (as returned by CacheKeyGenerator.key_for().
      :returns: True if the cached version of the item is out of date.