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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 get_purchase(self, purchase_id, purchase_key='sid'):
 """ Retrieve information about a purchase using the system's unique ID or a client's ID @param id_: a string that represents a unique_id or an extid. @param key: a string that is either 'sid' or 'extid'. """

        data = {'purchase_id': purchase_id,
                'purchase_key': purchase_key}
        return self.api_get('purchase', data)