code: def _validate_df(df: pd.DataFrame, *columns: str) -> None:
      null_columns = [
          column
          for column in columns
          if df[column].isnull().values.any()
      ]
      if null_columns:
          raise ValueError(
              f"Unable to transform columns {null_columns} because they contain "
              f"null values. Consider imputing missing values first."
          )
  
imports: from collections import Counter, OrderedDict
  from functools import partial
  from typing import Dict, List, Optional
  import numpy
  import pandas
  import pandas.api.types
  from ray.air.util.data_batch_conversion import BatchFormat
  from ray.data import Dataset
  from ray.data.preprocessor import Preprocessor, PreprocessorNotFittedException
  from ray.util.annotations import PublicAPI
label_desc: Use image node_id 2 for calling a global function with example usage: _validate_df(df) without return types