Re: How to forward a custom exception to Python

On Fri, Jan 31, 2020 at 05:14:49PM +0100, Lukáš Hrázký wrote:
> I'm having trouble defining a custom exception in a library and using
> it in Python bindings for the library generated by SWIG. I need to
> catch this exception in Python (as a user of the library) and handle it
> in a special way.

Hi Lukáš.

just some thoughts.

> * If I link the other modules against _error.so, and _error.so is in
>   LD_LIBRARY_PATH, it works. (this is not normally the case, so not a
>   good solution)

there is also runpath. if you have a (private/hidden) library in
/usr/lib/$yourpackage/liberror.so, you might be able to use it without
LD_LIBRARY_PATH. link with "--enable-new-dtags -rpath
/usr/lib/$yourpackage". (not sure how portable this is, but it should
work on linux).

> * If I link the other modules against _error.so and make sure the
>   mylib.error Python module is imported first, it works. (this is what
>   I have now, but the solution is a borderline hack and looks very
>   fragile)

make it look less like a hack...  _error.so does not have to be a
"module". It could be simpler to just put a single symbol into some
error.so, compiled from error.cc and linked with -shared.  you need to
include some simple header in both error.cc and _error.i.

> * If I don't link against _error.so and use:
>   import sys, os
>   sys.setdlopenflags(os.RTLD_NOW | os.RTLD_GLOBAL)

instead of linking all your modules against liberror, you could dlopen
the library from __init__.py using something like
ctypes.PyDLL("/some/liberror.so", mode=ctypes.RTLD_GLOBAL)

> How can I solve this? It seems to me like having a custom exception
> propagated to Python is a rather common use case and the "right" way to
> do error handling. I find it hard to believe I'm the first one to run
> into this.

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.

(not saying there is not a case for custom exceptions.)

> This could obviously work if I only had a single Python
> module with all the SWIG bindings. But I don't really want that and
> SWIG documentation also recommends multiple modules.

I don't know. having multiple modules may be the right way, but not
necessarily the easiest.

hth
felix


_______________________________________________
Swig-user mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/swig-user