def _retrieve_session(self, session_key):
session_id = session_key.session_id<EOL>if (session_id is None):<EOL><INDENT>msg = ("<STR_LIT>"<EOL>"<STR_LIT>"<EOL>"<STR_LIT>".format(session_key))<EOL>logger.debug(msg)<EOL>return None<EOL><DEDENT>session = self.session_store.read(session_id)<EOL>if (session is None):<EOL><INDENT>msg2 = "<STR_LIT>".format(session_id)<EOL>raise ValueError(msg2)<EOL><DEDENT>return session<EOL>
:type session_key: SessionKey
:returns: SimpleSession