code: def _add_new_columns(dataframe, metrics):
      
      #TODO(leodirac): we don't really need to do this on every update.  Optimize
      new_columns = set(metrics.keys()) - set(dataframe.columns)
      for col in new_columns:
          dataframe[col] = None
docs: Add new metrics as new columns to selected pandas dataframe.
  
      Parameters
      ----------
      dataframe : pandas.DataFrame
          Selected dataframe needs to be modified.
      metrics : metric.EvalMetric
          New metrics to be added.