class_docstr: Test the ClassificationScoreVisualizer base functionality
docstr: Assert warning and None returned if encoder doesn't have classes
docstr_tok: ["Assert","warning","and","None","returned","if","encoder","does","n't","have","classes"]
code: def test_labels_encoder_no_classes(self):
      """
      Assert warning and None returned if encoder doesn't have classes
      """
  
      class L2UTransformer(object):
          def transform(self, y):
              return np.array([yi.upper() for yi in y])
  
      oz = ClassificationScoreVisualizer(
          GaussianNB(), encoder=L2UTransformer()
      )
      with pytest.warns(
          YellowbrickWarning, match="could not determine class labels"
      ):
          assert oz._labels() is None
  
imports: import pytest
  import numpy
  import numpy.testing
  from yellowbrick.classifier.base import *
  from sklearn.naive_bayes import GaussianNB
  from sklearn.preprocessing import LabelEncoder
  from sklearn.linear_model import LinearRegression
  from .conftest import assert_fitted, assert_not_fitted
  from yellowbrick.exceptions import YellowbrickTypeError
label_desc: Use image node_id 13 for calling the TestClassificationScoreVisualizer obj's underlying member method code with example usage: obj.test_labels_encoder_no_classes() and returns: np