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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_category_activities(self, category_id = None):
 """Return activities for category. If category is not specified, will return activities that have no category"""

        category_id = category_id or -1
        return self._to_dict(('id', 'name', 'category_id', 'category'), self.conn.GetCategoryActivities(category_id))