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.
code: def __call__(self, *args, **kwargs):
      self._set_expected_value()
      return self
  
file_path: alibi/alibi/explainers/tests/utils.py
imports: import numpy
label_desc: Use image node_id 5 for calling the MockTreeExplainer obj's underlying member method code with example usage: obj.__call__() and returns: self