def __init__(self, relations=None, files=None):
        self._relations = {}

        if not atmost_one(relations, files):
            raise ValueError('Only one of relations and files should be given')

        if files:
            relations = self._initialize_relations(files)

        if relations:
            for relation in relations:
                self.add_relation(relation)
Constructor for the database class
dataset/ETHPy150Open coursera/dataduct/dataduct/database/database.py/Database.__init__