如何使用 Keras 神经网络分类器绘制 KFold 交叉验证中每个折叠的 ROC_AUC 曲线

Question

我真的需要使用 Keras ANN 在 5 折交叉验证中找到每个折的 ROC 图。我已经尝试了以下链接中的代码 [https://scikit-learn.org/stable/auto_examples/model_selection/plot_roc_crossval.html#sphx-glr-auto-examples-model-selection-plot-roc-crossval-py] [1] 当我使用 svm 分类器时，它工作得非常好，如此处所示。但是当我想使用包装器来使用 Keras ANN 模型时，它会显示错误。我已经坚持了几个月了。有人可以帮我吗？这是我的代码：

# Load libraries
import numpy as np
from keras import models
from keras import layers
from keras.wrappers.scikit_learn import KerasClassifier
from sklearn.model_selection import cross_val_score
from sklearn.datasets import load_breast_cancer
from sklearn.model_selection import StratifiedKFold

X, y = load_breast_cancer(return_X_y=True)
# Create function returning a compiled network
def create_network():
    
    # Start neural network
    network = models.Sequential()

    # Add fully connected layer with a ReLU activation function
    network.add(layers.Dense(units=16, activation='relu', input_shape=(30,)))

    # Add fully connected layer with a ReLU activation function
    network.add(layers.Dense(units=16, activation='relu'))

    # Add fully connected layer with a sigmoid activation function
    network.add(layers.Dense(units=1, activation='sigmoid'))

    # Compile neural network
    network.compile(loss='binary_crossentropy', # Cross-entropy
                    optimizer='rmsprop', # Root Mean Square Propagation
                    metrics=['accuracy']) # Accuracy performance metric
    
    # Return compiled network
    return network

cv = StratifiedKFold(n_splits=5)
# Wrap Keras model so it can be used by scikit-learn
classifier = KerasClassifier(build_fn=create_network, 
                                 epochs=10, 
                                 batch_size=100, 
                                 verbose=2)
#Plotting the ROC curve
tprs = []
aucs = []
mean_fpr = np.linspace(0, 1, 100)

fig, ax = plt.subplots()
for i, (train, test) in enumerate(cv.split(X, y)):
    classifier.fit(X[train], y[train])
    viz = plot_roc_curve(classifier, X[test], y[test],
                         name='ROC fold {}'.format(i),
                         alpha=0.3, lw=1, ax=ax)
    interp_tpr = np.interp(mean_fpr, viz.fpr, viz.tpr)
    interp_tpr[0] = 0.0
    tprs.append(interp_tpr)
    aucs.append(viz.roc_auc)

ax.plot([0, 1], [0, 1], linestyle='--', lw=2, color='r',
        label='Chance', alpha=.8)

mean_tpr = np.mean(tprs, axis=0)
mean_tpr[-1] = 1.0
mean_auc = auc(mean_fpr, mean_tpr)
std_auc = np.std(aucs)
ax.plot(mean_fpr, mean_tpr, color='b',
        label=r'Mean ROC (AUC = %0.2f $\pm$ %0.2f)' % (mean_auc, std_auc),
        lw=2, alpha=.8)

std_tpr = np.std(tprs, axis=0)
tprs_upper = np.minimum(mean_tpr + std_tpr, 1)
tprs_lower = np.maximum(mean_tpr - std_tpr, 0)
ax.fill_between(mean_fpr, tprs_lower, tprs_upper, color='grey', alpha=.2,
                label=r'$\pm$ 1 std. dev.')

ax.set(xlim=[-0.05, 1.05], ylim=[-0.05, 1.05],
       title="Receiver operating characteristic example")
ax.legend(loc="lower right")
plt.show()

它显示以下错误：

Epoch 1/10
5/5 - 0s - loss: 24.0817 - accuracy: 0.3714
Epoch 2/10
5/5 - 0s - loss: 2.7967 - accuracy: 0.5648
Epoch 3/10
5/5 - 0s - loss: 2.0594 - accuracy: 0.5363
Epoch 4/10
5/5 - 0s - loss: 2.4763 - accuracy: 0.5604
Epoch 5/10
5/5 - 0s - loss: 2.5489 - accuracy: 0.5121
Epoch 6/10
5/5 - 0s - loss: 2.0528 - accuracy: 0.6132
Epoch 7/10
5/5 - 0s - loss: 1.5593 - accuracy: 0.6088
Epoch 8/10
5/5 - 0s - loss: 2.0422 - accuracy: 0.5626
Epoch 9/10
5/5 - 0s - loss: 1.9191 - accuracy: 0.6242
Epoch 10/10
5/5 - 0s - loss: 1.9914 - accuracy: 0.5582
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-104-2d127e528fbe> in <module>()
      8     viz = plot_roc_curve(classifier, X[test], y[test],
      9                          name='ROC fold {}'.format(i),
---> 10                          alpha=0.3, lw=1, ax=ax)
     11     interp_tpr = np.interp(mean_fpr, viz.fpr, viz.tpr)
     12     interp_tpr[0] = 0.0

/usr/local/lib/python3.6/dist-packages/sklearn/metrics/_plot/roc_curve.py in plot_roc_curve(estimator, X, y, sample_weight, drop_intermediate, response_method, name, ax, **kwargs)
    170     )
    171     if not is_classifier(estimator):
--> 172         raise ValueError(classification_error)
    173 
    174     prediction_method = _check_classifer_response_method(estimator,

ValueError: KerasClassifier should be a binary classifier

Answer 1

这是这个包装库中（可能）缺少的一个实现细节。

Sklearn 只检查估计器上是否存在名为_estimator_type 的属性，并设置为字符串值classifier。您可以通过查看 github 上的 sklearn 的 source code 来了解这一点。

def is_classifier(estimator):
    """Return True if the given estimator is (probably) a classifier.
    Parameters
    ----------
    estimator : object
        Estimator object to test.
    Returns
    -------
    out : bool
        True if estimator is a classifier and False otherwise.
    """
    return getattr(estimator, "_estimator_type", None) == "classifier"

您需要做的就是手动将此属性添加到您的分类器对象中。

classifier = KerasClassifier(build_fn=create_network, 
                                 epochs=10, 
                                 batch_size=100, 
                                 verbose=2)

classifier._estimator_type = "classifier"

我已经测试过了，它可以工作。