Re: [Swig-devel] Custom exception handling in python

[Amaury F. d'A. <ama...@gm...>]

Hello,

2007/3/24, Olly Betts wrote:
> Replying to the swig-devel list, since this is discussing a change to
> SWIG itself...
>
> On 2007-03-09, Richard Boulton <ri...@le...> wrote:
> > I've recently converted a project using Swig (actually, Xapian) to use
> > custom exception types for the Python bindings, rather than using the
> > standard python exception types.  I've got this all working nicely,
> > except for one issue: I can't make (the Python proxy classes for) my
> > custom exception types inherit from the standard Python Exception class
> > (which all exceptions thrown in python should inherit from).
>
> Looking at the python docs, the nearest to a requirement I can find is
> section 8.5 here:
>
> http://docs.python.org/tut/node10.html
>
>     "Exceptions should typically be derived from the Exception class,
>     either directly or indirectly."
>
> To me "should typically" suggests a somewhat weaker requirement that RFC
> 2119's "SHOULD" (http://www.ietf.org/rfc/rfc2119.txt) though I don't
> see a situation where it's useful not to follow this recommendation.  It
> certainly seems that this is desirable default behaviour, though maybe
> there needs to be a way to override.

This is not only desirable, it will be enforced in future releases of python:
See http://www.python.org/dev/peps/pep-0352.

> > In the generated python code, I find the line:
> > "class Error:"
> >
> > and I would like to change this somehow to:
> > "class Error(Exception):"
> >
> > Is there any way to tell Swig that the proxy class for a C++ class
> > should inherit from a specific python class?  Or is there any other way
> > that I can get swig to do what I want?
>
> For Xapian, we could just run sed over the wrapper code which SWIG
> generates to fix this up, at least for now.
>
> I don't see a nice way to do it with SWIG.  It might be possible to
> lie to SWIG and claim the C++ Xapian::Error class inherits from an
> unspecified Exception class, but even if that works I think I prefer
> fixing up the generated code with sed.
>
> > (In theory, I wish that the %exceptionclass tag on a class would be
> > observed and used automatically to ensure that the corresponding proxy
> > class has the standard "Exception" class somewhere in it's ancestors -
> > but that's not currently implemented.  Perhaps that would be a useful
> > feature for python?)
>
> This sounds good to me.  What do the SWIG/Python experts think?
>

You could implement this with a __metaclass__. something like this:
(warning: not tested at all)

%pythoncode %{
class exceptionMetaclass(type):
   def __init__(self, name, bases, classdict):
       if Exception not in bases:
           bases.append(Exception)
       type.__init__(self, name, bases, classdict)
%}

class Xapian::Error
{
%pythoncode { __metaclass__ = exceptionMetaclass }
};

-- 
Amaury Forgeot d'Arc