def assertLoggedIn(self, d, username):
        """
        Assert that the L{Deferred} passed in is called back with the value
        'username'.  This represents a valid login for this TestCase.

        NOTE: To work, this method's return value must be returned from the
        test method, or otherwise hooked up to the test machinery.

        @param d: a L{Deferred} from an L{IChecker.requestAvatarId} method.
        @type d: L{Deferred}
        @rtype: L{Deferred}
        """
        result = []
        d.addBoth(result.append)
        self.assertEqual(len(result), 1, "login incomplete")
        if isinstance(result[0], Failure):
            result[0].raiseException()
        self.assertEqual(result[0], username)

        Assert that the L{Deferred} passed in is called back with the value
        'username'.  This represents a valid login for this TestCase.

        NOTE: To work, this method's return value must be returned from the
        test method, or otherwise hooked up to the test machinery.

        @param d: a L{Deferred} from an L{IChecker.requestAvatarId} method.
        @type d: L{Deferred}
        @rtype: L{Deferred}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/test/test_checkers.py