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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _add_new_columns(dataframe, metrics):
 """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. """

    #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