Re: How to forward a custom exception to Python

On Mon, 3 Feb 2020 at 09:57, Lukáš Hrázký <[email protected]> wrote:

> Hi Felix,
>
> On Sat, 2020-02-01 at 12:57 +0100, Felix Salfelder wrote:
> > On Fri, Jan 31, 2020 at 05:14:49PM +0100, Lukáš Hrázký wrote:
> > ...
> >
> > just some thoughts.
> >
> > ...
>
> Thanks for the multiple suggestions, at least I have options to
> consider.
>
> > > ...
> >
> > Perhaps one way is to map your exceptions to the corresponding python
> > style exceptions. eg raise a ValueError if some number is out of range,
> > whichever way your library signals this.
>
> Those standard exceptions have well-defined semantics and I need
> something else. I can't use them.
>
> > (not saying there is not a case for custom exceptions.)
>
> Yes :) I'd like to stress this is probably the biggest point of my
> email, custom exceptions are IMO simply _necessary_. Both my solutions
> and your suggestions are very involved, potentially fragile and non-
> transparent in case they break for a user of the library. At this stage
> it is also not clear to me which solution to pick, as they kind of look
> the same in the level of hackiness.
>
> Therefore I'd like to get some sort of statement with regards to this
> from the SWIG devs on the proper/recommended way or at least an opinion
> on the state of things. The problem is obviously hard to solve, but I'd
> still expect some sort of solution to this important aspect of wrapping
> an API...
>
>
Take a look at the two Python exception examples in the
Examples/python/exception and Examples/python/exceptproxy directories.
There is quite a bit of explanation, the highlights are the
'exceptionclass' feature which makes the wrapped class derive from the
Python Exception class. Note that if you use a C++ wrapped exception class
in a %catches list (see
http://swig.org/Doc4.0/SWIGPlus.html#SWIGPlus_catches) or in a C++98 style
exception specification list (see
http://swig.org/Doc4.0/SWIGPlus.html#SWIGPlus_exception_specifications)
then it is automatically marked with the 'exceptionclass' feature.

William

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