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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(self, identifier):
 """Fetch document by _id. Returns None if it is not found. (Will log a warning if not found as well. Should not be used to search an id.)"""

        logging.info('Download document with id ' + str(identifier) + '.')
        try:
            record = self.instance.get(index=self.index, doc_type=self.doc_type, id=identifier)
            if '_source' in record:
                return record['_source']
            else:
                return record
        except NotFoundError:
            return None