class_docstr: Module level tests for all sktime forecasters.
class_docstr_tok: ["Module","level","tests","for","all","sktime","forecasters","."]
docstr: Test that _y is updated when forecaster is refitted.
docstr_tok: ["Test","that","_y","is","updated","when","forecaster","is","refitted","."]
code: def test__y_when_refitting(self, estimator_instance, n_columns):
      """Test that _y is updated when forecaster is refitted."""
      y_train = _make_series(n_columns=n_columns)
      estimator_instance.fit(y_train, fh=FH0)
      estimator_instance.fit(y_train[3:], fh=FH0)
      # using np.squeeze to make the test flexible to shape differences like
      # (50,) and (50, 1)
      assert np.all(
          np.squeeze(estimator_instance._y) == np.squeeze(y_train[3:])
      )
  
imports: import numpy
  import pandas
  import pytest
  from sktime.datatypes import check_is_mtype
  from sktime.datatypes._utilities import get_cutoff
  from sktime.exceptions import NotFittedError
  from sktime.forecasting.base._delegate import _DelegatedForecaster
  from sktime.forecasting.base._fh import ForecastingHorizon
  from sktime.forecasting.tests._config import TEST_ALPHAS, TEST_FHS, TEST_OOS_FHS, TEST_STEP_LENGTHS_INT, TEST_WINDOW_LENGTHS_INT, VALID_INDEX_FH_COMBINATIONS
  from sktime.performance_metrics.forecasting import mean_absolute_percentage_error
  from sktime.split import ExpandingWindowSplitter, SlidingWindowSplitter, temporal_train_test_split
  from sktime.tests.test_all_estimators import BaseFixtureGenerator, QuickTester
  from sktime.utils._testing.forecasting import _assert_correct_columns, _assert_correct_pred_time_index, _get_expected_index_for_update_predict, _get_n_columns, _make_fh, make_forecasting_problem
  from sktime.utils._testing.series import _make_series
  from sktime.utils.validation.forecasting import check_fh
label_desc: Use image node_id 21 for calling the TestAllForecasters obj's underlying member method code with example usage: obj.test__y_when_refitting(estimator_instance, n_columns) without return types