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

[Stephan Houben]

Current documentation says:

"An object is compatible with an exception if it is the class or a base
class of the exception object or a tuple containing an item compatible with
the exception."

https://docs.python.org/3/reference/compound_stmts.html#the-try-statement

It is, in my opinion, not very clear from this that the __instancecheck__
mechanism is bypassed.

Should the documentation perhaps be adapted to explain that the class needs
to actually occur in the MRO
and that virtual base classes are not considered for matching purposes?

"An object is compatible with an exception if it is the class or a
non-virtual base class of the exception object or a tuple containing an
item compatible with the exception.
The exception matching machinery ignores the __instancecheck__ mechanism."


Stephan


2018-05-31 9:19 GMT+02:00 Terry Reedy <tjreedy at udel.edu>:

> 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
>
>
>
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