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From: Mike Bayer <mike_mp@zzzcomputing.com>
Date: Thu, 4 Oct 2018 10:18:08 -0400
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Subject: Re: Plans for 2.8
To: Daniele Varrazzo <daniele.varrazzo@gmail.com>
Cc: psycopg@lists.postgresql.org
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On Thu, Oct 4, 2018 at 8:38 AM Daniele Varrazzo
<daniele.varrazzo@gmail.com> wrote:
>
> Hello,
>
> next week I will have some time and maybe could end all the work to
> release psycopg 2.8. I am aware there are several features which have
> been awaiting for some time. The state of the work can be seen in [1].
>
> [1] https://github.com/psycopg/psycopg2/issues?q=milestone%3A"psycopg+2.8"
>
> Psycopg 2.8 will only support Python 2.7 and 3.4+. The codebase has
> been heavily hammered by Jon Dufresne who has killed a lot of Py2isms
> and the whole use of 2to3 is now replaced by a minimal compatibility
> module. So Python is now as modern as C in supporting both 2 and 3
> with a single codebase :P Other deprecated and unused objects have
> also been dropped, see the news file [2]. Fog, if you can take a look
> at examples/sandbox and delete what's no more required there (#645),
> that would be great :)
>
> [2] https://github.com/psycopg/psycopg2/blob/master/NEWS
>
> The feature I'm the most excited about (and worried about its
> reception) is to raise a different exception for every postgres error
> message (see #682) . For instance `SELECT * FROM wrong_name` will
> raise `UndefinedTable` rather than `ProgrammingError`. Currently
> handling a specific exception requires catching a broader class and
> looking at the pgcode:
>
>     try:
>         cur.execute("lock table %s in access exclusive mode nowait" % name)
>     except psycopg2.OperationalError as e:
>         if e.pgcode == psycopg2.errorcodes.LOCK_NOT_AVAILABLE:
>             locked = True
>         else:
>             raise
>
> This can become a much more natural:
>
>     try:
>         cur.execute("lock table %s in access exclusive mode nowait" % name)
>     except psycopg2.errors.LockNotAvailable:
>             locked = True
>
> The error classes are generated automatically from Postgres source
> code and are subclasses of the previously existing ones, so existing
> code should be unaffected. I'd be happy to have input about the
> feature and suggestions before releasing it.

I can't provide any suggestions, as the feature is very reasonable and
useful.   But I will lament that pep-249 has nothing about this, which
means from a driver-agnostic point of view, the situation is pretty
much unchanged.   Here's code I wrote for Openstack to try to apply
more specificity to database errors, basically a library of regexes:
https://github.com/openstack/oslo.db/blob/master/oslo_db/sqlalchemy/exc_filters.py#L55

one thing that would be helpful would be if your fine-grained
exception classes included more context about the failure.  Like
UndefinedTable would include the table name as an individual
datamember e.g. exception.table_name, an error about a foreign key
constraint would include the constraint name e.g.
exception.constraint_name, things like that.  You can see in my
oslo.db library above we are also pulling out other elements from the
error message to provide more context.


>
> A tiny improvement to SQL generation is already ready^W merged in
> #732: it will be possible to use `Identifier("schema", "name")` which
> would be rendered in dotted notation in the query. Currently
> `Identifier()` takes a single param so this extension is backward
> compatible and there is no need to introduce a new `Composable` type
> to represent dotted sequences of identifiers.
>
> There are requests to get extra info about the connection or the
> result (see #726, #661). They are reasonable and not too difficult to
> implement so I'd like to give them a go. However they are easy enough
> for someone to contribute if you feel? That would be very appreciated
> and would reduce the surface of the works to perform on my part.
> Another tiny feature would be to support IntEnum out-of-the-box
> (#591), which I've never used in Python.
>
> In the other thread these days we have discussed about introducing
> capsules: we can take a look to that too... Added #782.
>
> Thank you very much for any contribution, with ideas and even more with code :)
>
> -- Daniele
>