class_docstr: Base pairwise transformer for panel data template class.
  
  The base pairwise transformer specifies the methods and method signatures that all
  pairwise transformers have to implement.
  
  Specific implementations of these methods is deferred to concrete classes.
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docstr: Magic [...] method, return column subsetted transformer.
  
  Pairwise transformer (e.g., distance kernel) subsetted to the index.
  
  Keys must be valid inputs for `columns` in `ColumnSelect`.
  
  Parameters
  ----------
  key: valid input for `columns` in `ColumnSelect`, or pair thereof
      keys can also be a :-slice, in which case it is considered as not passed
  
  Returns
  -------
  the following TransformerPipeline object:
      ColumnSelect(columns) * self
      where `columns` only item in `key`
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code: def __getitem__(self, key):
      """Magic [...] method, return column subsetted transformer.
  
      Pairwise transformer (e.g., distance kernel) subsetted to the index.
  
      Keys must be valid inputs for `columns` in `ColumnSelect`.
  
      Parameters
      ----------
      key: valid input for `columns` in `ColumnSelect`, or pair thereof
          keys can also be a :-slice, in which case it is considered as not passed
  
      Returns
      -------
      the following TransformerPipeline object:
          ColumnSelect(columns) * self
          where `columns` only item in `key`
      """
      from sktime.transformations.series.subset import ColumnSelect
  
      return ColumnSelect(key) * self
  
imports: from sktime.base import BaseEstimator
  from sktime.datatypes import check_is_scitype, convert_to
  from sktime.datatypes._series_as_panel import convert_Series_to_Panel
label_desc: Use image node_id 6 for calling the BasePairwiseTransformerPanel obj's underlying member method code with example usage: obj.__getitem__(key) and returns: unknown