title: How to find true positives, true negatives, false positives, false negatives in Python
body: <p>I have trained a classifier in Python and i want to find the true positives, true negatives, false positives, false negatives when i am doing a new classification.
  The thing is that every time, my true_labels consist of one value points because in the problem i am investigating, i have only one label and i want to see how good the classifier performs on new data in recognizing this label. e.g:</p>
  
  <pre><code>labels_true = [2, 2, 2, 2, ..., 2]
  labels_predicted = [2, 2, 23, 2, 2, 2, 2, 21, ..., 2, 2, 2, 2]
  </code></pre>
  
  <p>Of course `len(labels_true)=len(labels_predicted). Since I have only one true label, how can i calculate the above metrics?</p>