from contextlib import contextmanager

class A:
    @contextmanager
    def transaction(self, isolation_level="DEFERRED"):
        prev_iso_lvl = self.conn.isolation_level
        self.conn.isolation_level = "DEFERRED"
        self.conn.execute("BEGIN")
        yield
        self.conn.commit()
        self.conn.isolation_level = prev_iso_lvl

    def insert_many(self, users, users_data):
        with self.transaction():
            for user in users:
                data = users_data[user['username']]
                self.insert(user, data)              # !!!

    def insert_many(self, users, users_data):
        with self.transaction():
            for user in users:
                data = users_data[user['username']]
                self.update(user, data)              # !!!