I'm trying to test how drivers will handle error cases within multi-statement transactions. I perform the following with my prototype of PyMongo that supports transactions:

session = client.start_session()
session.start_transaction()
collection.insert_one({'_id': 1}, session=session)
try:
    collection.insert_one({'_id': 1}, session=session)
except Exception as exc:
    print(exc)
collection.insert_one({'_id': 1}, session=session)
session.commit_transaction()

It appears (based on debugging) that the transaction includes the document {_id: 1} after the first insert, and the second insert not only causes a duplicate key error, it also aborts the transaction. Therefore the third insert succeeds (outside of any transaction), and the commit_transaction fails with code 125, errmsg "Transaction isn't in progress."