code: def _find_only_column_of_type(sframe, target_type, type_name, col_name):
      
      image_column_name = None
      if type(target_type) != list:
          target_type = [target_type]
      for name, ctype in zip(sframe.column_names(), sframe.column_types()):
          if ctype in target_type:
              if image_column_name is not None:
                  raise ToolkitError('No "{col_name}" column specified and more than one {type_name} column in "dataset". Can not infer correct {col_name} column.'.format(col_name=col_name, type_name=type_name))
              image_column_name = name
      if image_column_name is None:
          raise ToolkitError('No %s column in "dataset".' % type_name)
      return image_column_name
docs: Finds the only column in `SFrame` with a type specified by `target_type`.
      If there are zero or more than one such columns, an exception will be
      raised. The name and type of the target column should be provided as
      strings for the purpose of error feedback.