code: def _validate_feature_type(feature_type):
      """Transform feature type to an iterable and check that it exists."""
      if feature_type is None:
          feature_type_ = FEATURE_TYPE
      else:
          if isinstance(feature_type, str):
              feature_type_ = [feature_type]
          else:
              feature_type_ = feature_type
          for feat_t in feature_type_:
              if feat_t not in FEATURE_TYPE:
                  raise ValueError(
                      f"The given feature type is unknown. Got {feat_t} instead of one "
                      f"of {FEATURE_TYPE}."
                  )
      return feature_type_
  
file_path: scikit-image/skimage/feature/haar.py
imports: from itertools import chain
  from operator import add
  import numpy
  from ._haar import haar_like_feature_coord_wrapper
  from ._haar import haar_like_feature_wrapper
  from ..color import gray2rgb
  from ..draw import rectangle
  from ..util import img_as_float
label_desc: Use image node_id 1 for calling a global function with example usage: _validate_feature_type(feature_type) and returns: feature_type_