code: def _check_cv(cv_df: DataFrame) -> None:
      """
      Raise an error if cv_df does not exist
  
      Args:
          cv_df: Cross Validation DataFrame
      """
      if cv_df is None:
          raise ValueError(
              "No cross validation DataFrame found! Run fit() or provide a previously made cv DataFrame using set_cv_df()."
          )
  
imports: import itertools
  import math
  import re
  import warnings
  from typing import Any, Dict, Iterable, List, Tuple
  import numpy
  import pandas
  import pyspark.sql.functions
  import scipy.optimize
  from nptyping import Float, Int, NDArray
  from pyspark.sql import DataFrame, Row
  from pyspark.sql.window import Window
  from typeguard import typechecked
label_desc: Use image node_id 27 for calling a global function with example usage: _check_cv(cv_df) without return types