Re: Raising Python exceptions from C code

Hi Kevin

SWIG is wrapping C code where errors are not really expected in the target
language, SWIG usually expects some sort of C error handling and can deal
with that. Nevertheless, I think you can treat the setting of a Python
error as an equivalent C error setting mechanism and utilise %exception.
Here is a short example how:

%module example

%exception {
$action
if (PyErr_Occurred()) {
  printf("Failure!\n");
  SWIG_fail;
}
}
%inline %{
static int err = 0;
void * go(void) {
  PyErr_SetString(PyExc_RuntimeError, "Failed horribly");
  return NULL;
}
%}

This will generate code that will check for a Python error occurring
straight after the C function call and return straight away if one had.

William

On Wed, 8 Jul 2020 at 22:31, Kevin Smith <[email protected]>
wrote:

> I’m trying to figure out how I can raise a Python exception from a
> function in my .i file.  It’s fairly easy to call PyErr_SetString in the
> function, but the problem is that Python expects that when an exception
> occurs no value should be returned by the called function.  My function
> returns a pointer, but it’s NULL if an error occurs.  This NULL gets
> wrapped in a SWIG_NewPointerObj(SWIG_as_voidptr(result), … ) call and
> returned to Python which results in Python complaining about the returned
> value.  What I’d like to have happen in the wrapper is:
>
>    resultobj =  (!result && PyErr_Occurred()) ? NULL :
> SWIG_NewPointerObj(SWIG_as_voidptr(result), SWIGTYPE_p_void, 0 |  0 );
>
> This way, if the result is NULL and a Python exception happened during the
> function, a NULL would be returned to make Python happy.
>
> Maybe there’s a better way to do what I’m attempting, but this was what I
> came up with.  I’m just not sure how to get this syntax into my wrappers
> without post-processing them.
>
> Kevin Smith
> [email protected]
>
>
>
>
>
>
> _______________________________________________
> Swig-user mailing list
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