class_docstr: A class that can be used to replace shap.TreeExplainer so that
  the wrapper can be tested without fitting a model and instantiating
  an actual explainer.
docstr: Set random expected value for the explainer.
code: def _set_expected_value(self):
      """
      Set random expected value for the explainer.
      """
  
      if self.n_outputs == 1:
          self.expected_value = np.random.random()
      else:
          self.expected_value = [
              np.random.random() for _ in range(self.n_outputs)
          ]
  
file_path: alibi/alibi/explainers/tests/utils.py
imports: import numpy
label_desc: Use image node_id 4 for calling the MockTreeExplainer obj's underlying member method code with example usage: obj._set_expected_value() without return types