Re: [PATCH 17/20] python/qemu/qmp.py: Preserve error context on re-raise

[John Snow]

On 10/7/20 7:21 AM, Kevin Wolf wrote:
> Am 07.10.2020 um 01:58 hat John Snow geschrieben:
> > Use the "from ..." phrasing when re-raising errors to preserve their
> > initial context, to help aid debugging when things go wrong.
> >
> > This also silences a pylint 2.6.0+ error.
> >
> > Signed-off-by: John Snow <jsnow@redhat.com>
>
> I don't really understand what this improves compared to the implicit
> chaining we got before, but if pylint says so...
>
> Reviewed-by: Kevin Wolf <kwolf@redhat.com>

Yeah, it's a pretty minimal change. Depending on the context, I think it 
makes a bigger difference depending on how far away you are from the 
error you are re-raising, but I couldn't find a great real-world example 
for you right now.

In summary, it changes this line:

"During handling of the above exception, another exception occurred:"

to this one:

"The above exception was the direct cause of the following exception:"

Which disambiguates between wrapping an exception with a more 
semantically meaningful exception class, vs. your handler code itself 
faulted.

Minor change, I know. You are also allowed to use "from None" to 
suppress the chain. I use this in the QAPI series at one point because I 
felt the underlying error was not useful to see in the traceback.

I see the pylint change as forcing you not to rely on the implicit 
chaining. Eh, fine.

--js