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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code: def __iter__(self):
      return iter(self.label_list)
  
imports: import warnings
  import numpy
  import pandas
  from typing import Any, Hashable, List, Optional, Tuple, Type, Union
label_desc: Use image node_id 3 for calling the _ListLabel obj's underlying member method code with example usage: obj.__iter__() and returns: iter