code: def get_search_key_column(self, df: pd.DataFrame) -> Optional[str]:
      maybe_column = self._get_search_key_by_name(df.columns.to_list())
      if maybe_column is not None:
          return maybe_column
  
      for column_name in df.columns:
          if self._is_search_key_by_values(df[column_name]):
              return column_name
  
imports: from typing import List, Optional
  import pandas
label_desc: Use image node_id 4 for calling the BaseSearchKeyDetector obj's underlying member method code with example usage: obj.get_search_key_column(df) and returns: maybe_column, column_name