code: def _read_col_index(self, qual, step):
      try:
          return self._col_index_map[(qual or "last", step)]
      except KeyError:
          raise ValueError(
              f"unsupported scalar type qual={qual!r} step={step}"
          ) from None
  
imports: import logging
  import os
  import sqlite3
  from guild import run_util
  from guild import tfevent
  from guild import util
  from guild import var
label_desc: Use image node_id 10 for calling the ScalarReader obj's underlying member method code with example usage: obj._read_col_index(qual, step) and returns: self