class_docstr: An helper class that handles a list of labels.
  
  The class is to be used in the `DFtoVW` class. It parses the list of labels
  that the user could provide in the `label` argument of `DFtoVW`. The class only accepts
  a pre-defined set of label classes (defined in the class attribute `available_labels`).
  
  Args:
      label_list: The list of labels that the user passed in the attribute label of DFtoVW.
  
  Raises:
      ValueError: If the list passed has mixed types or if the labels should not be used in a list.
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docstr: Return the string representation of the labels of the underlying list, separated by a pre-defined character.
  
  Args:
      df: The dataframe from which to select the columns.
  
  Returns:
      The _ListLabel string representation.
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code: def process(self, df: pd.DataFrame) -> pd.Series:
      """Return the string representation of the labels of the underlying list, separated by a pre-defined character.
  
      Args:
          df: The dataframe from which to select the columns.
  
      Returns:
          The _ListLabel string representation.
      """
      for i, label in enumerate(self):
          if i == 0:
              out = label.process(df)
          else:
              out += self.sep + label.process(df)
  
      return out
  
imports: import warnings
  import numpy
  import pandas
  from typing import Any, Hashable, List, Optional, Tuple, Type, Union
label_desc: Use image node_id 5 for calling the _ListLabel obj's underlying member method code with example usage: obj.process(df) and returns: out