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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 variables(self, include_units=False):
 """Get a list of variables Parameters include_units: boolean, default False include the units """

        if include_units:
            return self.data[['variable', 'unit']].drop_duplicates()\
                .reset_index(drop=True).sort_values('variable')
        else:
            return pd.Series(self.data.variable.unique(), name='variable')