code: def _evaluate_usecols(usecols, names):
      
      if callable(usecols):
          return {i for i, name in enumerate(names) if usecols(name)}
      return usecols
docs: Check whether or not the 'usecols' parameter
      is a callable.  If so, enumerates the 'names'
      parameter and returns a set of indices for
      each entry in 'names' that evaluates to True.
      If not a callable, returns 'usecols'.