python - roc_auc_score - Only one class present in y_true

Question

I am doing a k-fold XV on an existing dataframe, and I need to get the AUC score. The problem is - sometimes the test data only contains 0s, and not 1s!

I tried using this example, but with different numbers:

import numpy as np
from sklearn.metrics import roc_auc_score
y_true = np.array([0, 0, 0, 0])
y_scores = np.array([1, 0, 0, 0])
roc_auc_score(y_true, y_scores)

And I get this exception:

ValueError: Only one class present in y_true. ROC AUC score is not defined in that case.

Is there any workaround that can make it work in such cases?

Answer 1

You could use try-except to prevent the error:

import numpy as np
from sklearn.metrics import roc_auc_score
y_true = np.array([0, 0, 0, 0])
y_scores = np.array([1, 0, 0, 0])
try:
    roc_auc_score(y_true, y_scores)
except ValueError:
    pass

Now you can also set the roc_auc_score to be zero if there is only one class present. However, I wouldn't do this. I guess your test data is highly unbalanced. I would suggest to use stratified K-fold instead so that you at least have both classes present.