Hello PyGreSQL,
As per <http://www.postgresql.org/files/developer/concurrency.pdf> (slide 25,
"So you end up needing a retry loop anyway"), pretty much any concurrent
PostgreSQL application requires a try/except retry loop somewhere. But, a
blanket catch-all retry loop won't be quite right; in many cases, concurrency
issues should only cause very specific errors and require specific responses to
those error types. For example, if you have a table that might be concurrently
INSERTed into, you might want to handle 23505, "UNIQUE VIOLATION", but allow
any other error to bubble up and be reported as an unexpected exception, with a
logged traceback and all.
The Postgres documentation helpfully enumerates all of these errors
(<http://www.postgresql.org/docs/8.4/static/errcodes-appendix.html>) and libpq
provides a function to access the error code in a structured,
non-localization-sensitive way (PQresultErrorField(res, PG_DIAG_SQLSTATE)
http://www.postgresql.org/docs/8.4/static/libpq-exec.html#AEN33971>) but it
appears that PyGreSQL doesn't expose this information.
cx_Oracle provides similar information in its '.code' attribute:
<http://cx-oracle.sourceforge.net/html/module.html#exception-handling>.
psycopg2 provides exactly this information in its '.pgcode' attribute and named
constants to compare against in its 'errorcodes' module:
<http://initd.org/psycopg/docs/errorcodes.html#module-psycopg2.errorcodes>.
So, despite the fact that this isn't technically standardized in DB-API 2.0, it
seems like it is a commonly implemented extension and it would be very nice to
have.
Personally my preference would be to actually have a separate exception class
for each error code, so that I could do 'except pg.errors.UniqueViolation:',
but that is probably a bunch more work than simply adding an integer attribute.
Would you please add a similar attribute for the next PyGreSQL release? If
there's somewhere else that would be more appropriate to submit this feature
request, please let me know.
Thanks in advance for your consideration.
-glyph
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