clarification on finalization

[euri10]

greetings, I first wrote this snippet, which appears to be an incorrect way of dealing with finalization:

@get("/foo")
async def foo(db_pool: Pool, db_connection: Connection) -> Stream:
    stream_id = await db_connection.fetchval(
        """insert into toto(stream) values ('') returning id"""
    )
    print(stream_id)

    async def chunks(num, stream_id):
        async with db_pool.acquire() as conn:
            try:
                for i in range(num):
                    chunk = f"chunk #{i}\n"
                    yield chunk
                    await asyncio.sleep(0.5)
                    update = await conn.execute(
                        f"update toto set stream = '{i}' where id={stream_id}"
                    )
                    print(update)
            except CancelledError as e:
                print(str(e))
                delete = await conn.execute(f"delete from toto where id={stream_id}")
                print(delete)
		raise
            finally:
                print("end")

    return Stream(chunks(10, stream_id))

then I read the docs which says:

If you need to use await during finalization, you need to enclose it in a shielded cancel scope, or the operation will be cancelled immediately since it’s in an already cancelled scope:

so the "correct" way to write this would be to do:

            except CancelledError as e:
                print(str(e))
                with CancelScope(shield=True):
                    delete = await conn.execute(f"delete from toto where id={id}")
                    print(delete)
		raise
            finally:
                print("end")

and it indeed prints DELETE 1 and effectively removes the entry fro the database.

however, it appears that the incorrect way of dealing with an await inside the exception deletes the database entry and doesnt print DELETE 1.

from the docs description I would have expected the delete NOT to be performed, so is there some part of the puzzle I'm missing here ?

[agronholm]

It all depends on what exactly conn.execute() does. It may or may not work, but gist is that if CancelledError was raised by an AnyIO cancel scope, then it will attempt to cancel the task over and over again until that CancelledError reaches the cancel scope that initiated the cancellation process.

[agronholm]

This is what CancelScope(shield=True): is for: it prevents that constant cancellation by AnyIO cancel scopes so that the code in conn.execute() can run without getting hit by those constant cancellations.

[euri10]

conn.execute is a simple asyncpg call, so if I understand your point it's working by accident in the case I didnt shield it, should the call to the db be "slower" then cancellation would happen before.

thanks for the clarification

[smurfix]

Could you please edit your question to fix the broken indent on the raise lines?

[euri10]

ok this explains the misunderstanding,
it's very worrying from github to say the least :)

[euri10]

just checked on firefox, which is the same for me as chrome though a little bit less indented, both on linux, could be because of a mix of tabs and spaces ?

anyways, that's not really the topic, but very confusing, both browsers are default one, no customization, stock debian bullseye,

[agronholm]

I don't know what to say, except that I also checked on Windows (both Edge and Firefox) and the indent was rendered exactly the same there too.

[smurfix]

could be because of a mix of tabs and spaces ?

Most probably. Lesson learned: Don't paste tabs.

[euri10]

ok sorry guys for this, i found the culprit in fact I'm an extension called Refined Github and removing it I see what I'm supposed to see 😌

https://chromewebstore.google.com/detail/refined-github/hlepfoohegkhhmjieoechaddaejaokhf