def _userLogin(self):
        """
        Authenticates the FTP client using the test account.

        @return: L{Deferred} of command response
        """
        d = self.assertCommandResponse(
            'USER %s' % (self.username),
            ['331 Password required for %s.' % (self.username)])
        return self.assertCommandResponse(
            'PASS %s' % (self.password),
            ['230 User logged in, proceed'],
            chainDeferred=d)

        Authenticates the FTP client using the test account.

        @return: L{Deferred} of command response
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_ftp.py