def get_num_examples(self):
        raise NotImplementedError()
Called when our 'visible' trait is changed.
dataset/ETHPy150Open lisa-lab/pylearn2/pylearn2/datasets/dataset.py/Dataset.get_num_examples--crossover with--dataset/ETHPy150Open enthought/pyface/pyface/ui/wx/action/action_item.py/_MenuItem._visible_changed