The test generator uses the same source code for train and valid generator, where the parent folder will be treated as the class name, but in case of test data the generator expects the same structure so a dummy parent folder is needed, which will be treated as a single class (which we will ignore by setting class_mode=None). The Evaluate generator can be used only if you have both the data and the respective target values, the test generator will not have target values, to begin with.