class_docstr: Simple selector that subsets a DataFrame to the columns of interest
  This allows a Pipeline that applies different transformations to subsets of columns,
     and unions that result
code: def transform(self, X):
      assert isinstance(X, pd.DataFrame)
      return X[self.column_names]
  
file_path: greykite/greykite/sklearn/transform/column_selector.py
imports: import pandas
  from sklearn.base import BaseEstimator
  from sklearn.base import TransformerMixin
label_desc: Use image node_id 3 for calling the ColumnSelector obj's underlying member method code with example usage: obj.transform(X) and returns: X