class_docstr: The abstract base class `BaseEstimator` defines the basic requirements of an estimator in ART. The BaseEstimator is
  is the highest abstraction of a machine learning model in ART.
class_docstr_tok: ["The","abstract","base","class","`","BaseEstimator","`","defines","the","basic","requirements","of","an","estimator","in","ART",".","The","BaseEstimator","is","is","the","highest","abstraction","of","a","machine","learning","model","in","ART","."]
code: def _set_preprocessing_defences(
      preprocessing_defences: Optional[
          Union["Preprocessor", List["Preprocessor"]]
      ]
  ) -> Optional[List["Preprocessor"]]:
      from art.defences.preprocessor.preprocessor import Preprocessor
  
      if isinstance(preprocessing_defences, Preprocessor):
          return [preprocessing_defences]
  
      return preprocessing_defences
  
imports: from abc import ABC, abstractmethod
  from typing import Any, Dict, List, Optional, Tuple, Union, TYPE_CHECKING
  import numpy
  from tqdm.auto import trange
  from art.config import ART_NUMPY_DTYPE
label_desc: Use image node_id 4 for calling the BaseEstimator obj's underlying member method code with example usage: obj._set_preprocessing_defences(preprocessing_defences) and returns: preprocessing_defences, list