code: def test_no_labels_at_50(self):
      y_true = np.array(
          [nan, nan, nan, nan, nan, nan, nan, nan, nan, nan]
      )
      y_score = np.random.rand(1, 10)
      labels = labels_at(
          y_true, y_score, top_proportion=0.5, normalize=False
      )
      self.assertEqual(labels, 0)
  
imports: from unittest import TestCase
  from sklearn_evaluation.metrics import precision_at, labels_at, tp_at, fp_at
  import numpy
  from numpy import nan
label_desc: Use image node_id 2 for calling the Test_labels_at obj's underlying member method code with example usage: obj.test_no_labels_at_50() without return types