Return a cache bucket for the given template.  All arguments are
        mandatory but filename may be `None`.

def get_bucket(self, environment, name, filename, source):
        """Return a cache bucket for the given template.  All arguments are
        mandatory but filename may be `None`.
        """
        key = self.get_cache_key(name, filename)
        checksum = self.get_source_checksum(source)
        bucket = Bucket(environment, key, checksum)
        self.load_bytecode(bucket)
        return bucket