mask_acc in keras (keras the unmarked category not assess the accuracy)

import keras.backend as K

def no_ground_accuracy(y_true, y_pred):
    y_true = K.argmax(y_true, axis=-1)
    y_pred = K.argmax(y_pred, axis=-1)

    mask = K.cast(K.not_equal(y_true, 15), K.floatx())  # 15是未标记的类别标签
    total = K.sum(mask)
    total_correct = K.cast(K.equal(y_true, y_pred), K.floatx())
    correct = K.sum(total_correct * mask)

    return correct / total