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

6.py

class LockManager:
    def __init__(self):
        self.locks = []
        
    def add(self, transaction, record_id):
        if not self.exists(transaction, record_id):
            self.locks.append([transaction, record_id])
            
    def exists(self, transaction, record_id):
        return any(lock[0] is transaction and lock[1] == record_id \
            for lock in self.locks)