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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 clear(self, train=False, split=0):
 """Delete Features of each class from the database. :param train: Whether or not to clear the FeatureKeys :type train: bool :param split: Which split of candidates to clear features from. :type split: int """

        # Clear Features for the candidates in the split passed in.
        logger.info(f"Clearing Features (split {split})")

        sub_query = (
            self.session.query(Candidate.id).filter(Candidate.split == split).subquery()
        )
        query = self.session.query(Feature).filter(Feature.candidate_id.in_(sub_query))
        query.delete(synchronize_session="fetch")

        # Delete all old annotation keys
        if train:
            logger.debug(f"Clearing all FeatureKeys from {self.candidate_classes}...")
            drop_all_keys(self.session, FeatureKey, self.candidate_classes)