https://medium.com/@elliotchance/implementing-repeatable-read-and-serializable-transaction-isolation-8b806a14026e

4.py

class ReadCommittedTransaction(Transaction):
    def record_is_locked(self, record):
        return record['expired_xid'] != 0 and \
            row['expired_xid'] in self.table.active_xids
      
    def record_is_visible(self, record):
        # The record was created in active transaction that is not our
        # own.
        if record['created_xid'] in self.table.active_xids and \
            record['created_xid'] != self.xid:
            return False
          
        # The record is expired or and no transaction holds it that is
        # our own.
        if record['expired_xid'] != 0 and \
            (record['expired_xid'] not in self.table.active_xids or \
            record['expired_xid'] == self.xid):
            return False
          
        return True