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

1.py

class Transaction:
    def __init__(self, table, xid):
        self.table = table
        self.xid = xid
        self.rollback_actions = []
        
    def add_record(self, id, name):
        record = {
            'id': id,
            'name': name,
            'created_xid': self.xid,
            'expired_xid': 0
        }
        self.rollback_actions.append(["delete", len(self.table.records)])
        self.table.records.append(record)
        
    def delete_record(self, id):
        for i, record in enumerate(self.table.records):
            if self.record_is_visible(record) and record['id'] == id:
                if self.record_is_locked(record):
                    raise RollbackException("Row locked by another transaction.")
                else:
                    record['expired_xid'] = self.xid
                    self.rollback_actions.append(["add", i])
                    
    def update_record(self, id, name):
        self.delete_record(id)
        self.add_record(id, name)
        
    def fetch_record(self, id):
        for record in self.table.records:
            if self.record_is_visible(record) and record['id'] is id:
                return record
              
        return None
      
    def count_records(self, min_id, max_id):
        count = 0
        for record in self.table.records:
            if self.record_is_visible(record) and \
                min_id <= record['id'] <= max_id:
                count += 1
                
        return count
      
    def fetch_all_records(self):
        visible_records = []
        for record in self.table.records:
            if self.record_is_visible(record):
                visible_records.append(record)
                
        return visible_records
      
    def fetch(self, expr):
        visible_records = []
        for record in self.table.records:
            if self.record_is_visible(record) and expr(record):
                visible_records.append(record)
                
        return visible_records
      
    def commit(self):
        self.table.active_xids.discard(self.xid)
        
    def rollback(self):
        for action in reversed(self.rollback_actions):
            if action[0] == 'add':
                self.table.records[action[1]]['expired_xid'] = 0
            elif action[0] == 'delete':
                self.table.records[action[1]]['expired_xid'] = self.xid
                
        self.table.active_xids.discard(self.xid)