sklearn LogisticRegression only predicts 1, but predict_proba has many values

Question

I am getting a strange output from sklearn's LogisticRegression, where my trained model classifies all observations as 1s.

In [1]:
logit = LogisticRegression(C=10e9, random_state=42)
model = logit.fit(X_train, y_train)
classes = model.predict(X_test)
probs = model.predict_proba(X_test)

print np.bincount(classes)

Out [1]: 
[   0 2458]

But look at the predicted probabilities:

How is this possible?

I know that there is another post on this (here), but it does not answer this question. I understand that my classes are not balanced (this uniform classification goes away when I enter the argument class_weights = balanced).

However, I want to understand why sklearn is classifying predicted probabilities of less than 0.5 as a positive event.

Thoughts?

Answer 1

Notice how your plot is symmetric? That's because predict_proba has shape (n_samples, n_classes), so half the data you've plotted is redundant with the other half (since $p_i + (1 - p_i) = 1$).

If you look at probs[:,1] by itself I'm sure it will make sense.