code: def test_is_fitted(self):
      params = {"max_depth": 1, "seed": 1, "ml_task": "regression"}
      model = DecisionTreeRegressorAlgorithm(params)
      self.assertFalse(model.is_fitted())
      model.fit(self.X, self.y)
      self.assertTrue(model.is_fitted())
  
file_path: mljar-supervised/tests/tests_algorithms/test_decision_tree.py
imports: import unittest
  import tempfile
  import json
  import numpy
  import pandas
  import os
  from numpy.testing import assert_almost_equal
  from sklearn import datasets
  from supervised.algorithms.decision_tree import DecisionTreeAlgorithm, DecisionTreeRegressorAlgorithm
  from supervised.utils.metric import Metric
  import tempfile
label_desc: Use image node_id 4 for calling the DecisionTreeTest obj's underlying member method code with example usage: obj.test_is_fitted() without return types