why does query invoke a transaction

mysqlclient==1.4.2.post1 SQLAlchemy==1.3.5

def test4():
    Session = db.session  # db.session = sessionmaker(bind=self.engine, **session_options)

    print("start")
    print(db.status())
    conn = db.engine.connect()
    print(db.status())
    print("after check out one connection")

    session = Session(bind=conn)
    new_user = User()
    new_user.id = 3
    print("begin the transaction")

    session.query(User).filter_by(id=1).first()


    # session.rollback() # uncomment this line. everything works fine
    print("invalidate the connection. mock the connection not available")
    conn.invalidate()

    print(db.status())
    session.query(User).filter_by(id=1).first()

    print(db.status())

this will generate exception:

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1177, in _execute_context
    conn = self._revalidate_connection()
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 463, in _revalidate_connection
    "Can't reconnect until invalid "
sqlalchemy.exc.InvalidRequestError: Can't reconnect until invalid transaction is rolled back

After this, this connection is no longer usable anymore.

My question is : Why does query invoke a transaction? Normally speaking, if there is no transaction, conn.invalidate() won’t cause any problems. It just close the connection and return it back to pools, right? For example, if add session.rollback() right after query, everything works just fine.

[chenyuan53618]

Thx, @zzzeek. I just became your patron. Really worth all detailed answers

[zzzeek]

closing this for now.