[Python-ideas] Let try-except check the exception instance

[Terry Reedy]

On 5/31/2018 12:47 AM, Danilo J. S. Bellini wrote:
> Hi!
> I was working on handling some exceptions from external software
> (e.g. database constraint triggers)
> switching the handler based on the messages that had been sent.
> Today we can do something like (running on Python 3.6.5):
> 
> 
>>>> try:
> ...     # [...]
> ...     session.commit() # Here it raises!
> ...     # [...]
> ... except DatabaseError as exc:
> ...     msg = get_db_error_msg_from_exception(exc)
> ...     if msg == "beyond_limit":
> ...         # [...]
> ...     elif msg == "no_funds":
> ...         # [...]
> ...     else:
> ...         raise
> 
> 
> That works,

Yes, it works perfectly well, AND it exposes the fact that your code 
depends on the message, which I think is a good thing.

As Stephen said, messages are intentionally not part of the defined API. 
  As a matter of curtesy, we usually restrict message changes to new 
versions and do not backport the change. An exception may be made if we 
decide that a message is sufficiently erroneous that is likely misleads 
people.  In any case, message dependent code may be version dependent.

-- 
Terry Jan Reedy