def getInvalidAuthType():
    """
    Helper method to produce an auth type that doesn't exist.
    """
    invalidAuthType = 'ThisPluginDoesNotExist'
    while (invalidAuthType in
           [factory.authType for factory in strcred.findCheckerFactories()]):
        invalidAuthType += '_'
    return invalidAuthType

    Helper method to produce an auth type that doesn't exist.
    
stackoverflow/venv/lib/python3.6/site-packages/twisted/cred/test/test_strcred.py