import pymongo

mgClient = pymongo.MongoClient("ip", "port")

session = mgClient.start_session()

a_collection = mgClient.get_database("库").集合名a

b_collection = mgClient.get_database("库").集合名b

# 事务开始

session.start_transaction()

try:

    a_collection.insert({"world":1})

    b_collection.insert({"hello":1})

except:

    # 操作异常，中断事务

    session.abort_transaction()

else:

    session.commit_transaction()

finally:

    session.end_session()