code: def _get_type_tuple(estimator_scitype):
      """Convert scitype string(s) into tuple of classes for isinstance check.
  
      Parameters
      ----------
      estimator_scitypes: None, string, or list of string
  
      Returns
      -------
      estimator_classes : tuple of sktime base classes,
          corresponding to scitype strings in estimator_scitypes
      """
      if estimator_scitype is not None:
          estimator_classes = tuple(
              BASE_CLASS_LOOKUP[scitype]
              for scitype in _to_list(estimator_scitype)
          )
      else:
          estimator_classes = (BaseObject,)
  
      return estimator_classes
  
file_path: sktime/sktime/registry/tests/test_lookup.py
imports: import pytest
  from sktime.base import BaseObject
  from sktime.registry import all_estimators, all_tags, scitype
  from sktime.registry._base_classes import BASE_CLASS_LOOKUP, BASE_CLASS_SCITYPE_LIST, TRANSFORMER_MIXIN_SCITYPE_LIST
  from sktime.registry._lookup import _check_estimator_types
label_desc: Use image node_id 2 for calling a global function with example usage: _get_type_tuple(estimator_scitype) and returns: estimator_classes