gistfile1.txt

    def GetCacheKey(self, descriptor_files):
        descriptor = None
        for f in descriptor_files:
            if self.ctx.Contains(f):
                descriptor = f
                descriptor_contents = self.ctx.GetFile(descriptor)
                break
        if not descriptor:
            logging.info('No package descriptor found. No packages installed.')
            return None
        return hashlib.sha256(descriptor_contents).hexdigest()


    def GetCachedImageIfExists(self, checksum):
        # def GetCachedImageIfExists(self, base_image, cache, use_cache=True):
        if not checksum:
            # TODO(aaron-prindle) verify this makes sense to use None
            # as sentinel for no descriptor and to handle this here
            return base_image

        hit = cache.Get(base_image, _PYTHON_NAMESPACE, checksum)
        if hit:
            logging.info('Found cached dependency layer for %s' % checksum)
            last_created = _timestamp_to_time(_creation_time(hit))
            now = datetime.datetime.now()
            if last_created > now - datetime.timedelta(
                    seconds=_DEFAULT_TTL_WEEKS):
                return hit
            else:
                logging.info('TTL expired for cached image, rebuilding %s'
                            % checksum)
        else:
            logging.info('No cached dependency layer for %s' % checksum)
        return None