Re: proposal: PL/Pythonu - function ereport

2016-01-26 8:22 GMT+01:00 Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>:

>
>
> 2016-01-26 7:31 GMT+01:00 Catalin Iacob <iacobcatalin(at)gmail(dot)com>:
>
>> On 1/21/16, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
>> > 2016-01-14 17:16 GMT+01:00 Catalin Iacob <iacobcatalin(at)gmail(dot)com>:
>> >> Consider this call chain: SQL code 1 -> PL/Python code 1 -> SPI (via
>> >> plpy.execute) -> SQL code 2 -> PL/Python code 2
>> >>
>> >> If I'm writing PL/Python code 1 and I want to raise an error toward
>> >> SQL code 1 I use raise plpy.Error. plpy.SPIError is what I get if I
>> >> call into SQL code 2 and that has an error. That error could indeed
>> >> come from a plpy.Error thrown by PL/Python code 2 or it could come
>> >> from a SQL syntax error or whatever. But the symmetry holds:
>> >> plpy.Error is what you raise to stop the query and return errors to
>> >> your SQL caller and plpy.SPIError is what you get back if you use SPI
>> >> to execute some other piece of SQL which has an error. I *think* (I'm
>> >> an internals newbie though so I could be wrong) that SQL code 1
>> >> doesn't call into PL/Python code 1 via SPI so why would the latter use
>> >> something called SPIError to inform the former about an error?
>> >>
>> >
>> > It is not correct - outside PLPython you got a Error (PostgreSQL error
>> has
>> > not any classes), and isn't important the raising class (Error or
>> > SPIError). Inside PL/Python you will got SPIError or successors (based
>> on
>> > SQLcode).
>>
>> What exactly is not correct? Indeed, *outside* PLPython you don't get
>> a Python exception because Postgres isn't Python, you get Postgres'
>> way of representing an exception (error code, message, hint and so on
>> that might go to the client or not etc.). But that's normal, it's just
>> the impedance mismatch between the fact that you embed Python with its
>> rules into Postgres with its other rules. It's normal that the "host"
>> (Postgres) wins in the end and uses its mechanism to report errors.
>> The "guest" (PLPython) is there only to give mechanisms to activate
>> the "host".
>>
>> But *inside* PLPython what I wrote is true, see this example for what I
>> mean:
>>
>> CREATE FUNCTION test()
>> RETURNS int
>> AS $$
>> def a_func():
>> raise plpy.Error('an error')
>>
>> try:
>> a_func()
>> except plpy.Error as exc:
>> plpy.info('have exc {} of type {}'.format(exc, type(exc)))
>> plpy.info('have spidata {}'.format(hasattr(exc, 'spidata')))
>> $$ LANGUAGE plpython3u;
>>
>> Running this produces:
>>
>> DROP FUNCTION
>> CREATE FUNCTION
>> psql:plpy_demo:16: INFO: have exc an error of type <class 'plpy.Error'>
>> psql:plpy_demo:16: INFO: have spidata False
>>
>> > Currently If I raise plpy.Error, then it is immediately trasformed to
>> > PostgreSQL, and and then to SPIError and successors.
>>
>> Depends how you define immediately. I would say it's really not
>> immediately, it's only at the Postgres boundary. Imagine in the
>> example above that a_func could call lots of other Python code and
>> somewhere deep down raise Error would be used. Within that whole
>> execution the error stays Error and can be caught as such, it will
>> have nothing to do with SPIError.
>>
>> But in my opinion this discussion shouldn't really even be about
>> catching these things, most of the times you won't catch them and
>> instead you'll let them go to Postgres. The discussion should be
>> whether raise plpy.Error(...), plpy.raise_error, plpy.raise_info(,,,)
>> etc. all with keyword argument support are a good PLPython interface
>> to Postgres' ereport. I think they are.
>>
>> On a different note, I've explained what I think but I might be wrong
>> and don't want to stall the patch just because I don't agree with the
>> approach it takes.
>>
>> The waiting for author status is ok since, as far as I can see, Pavel
>> also agrees that at least some of the issues raised in
>>
>> http://www.postgresql.org/message-id/CAHg_5grU=LVRbZDhfCiiYc9PhS=1X5f=Gd44-3JcuL-QAoRC-A@mail.gmail.com
>> need to be fixed.
>>
>> Would it help maybe to remove myself as reviewer in the CF? Maybe that
>> makes somebody else pick it up and move it further.
>>
>
please, wait. We can finish the part, where is a agreement. And although it
is not final solution from Python perspective - I hope, so it can be really
useful for PLPython user - there will be one way how to raise rich
exception without ugly workaround

Regards

Pavel

>
>> I could also code the version I'm describing but that design would be
>> contrary to what Jim and Pavel lean towards now. It could help to
>> compare approaches side by side but I don't like the idea of dueling
>> patches.
>>
>
> I would to reduce this patch and don't try to touch on buildin exceptions.
> I hope, so there isn't any disagreement?
>
> I though about it lot of, and I see a the core of problems in orthogonal
> constructed exceptions in Python and Postgres. We working with statements
> elog, ereport, RAISE EXCEPTION - and these statements are much more like
> templates - can generate any possible exception. Python is based on working
> with instances of predefined exceptions. And it is core of problem.
> Template like class cannot be ancestor of instance oriented classes. This
> is task for somebody who knows well OOP and meta OOP in Python - total
> different chapter, different patch.
>

>

> Regards
>
> Pavel
>
>
>