#39 example asserts with python3.4-dbg

[Julian Taylor]

the example.cxx of pycxx crashes when run with python3.4 in debug mode.
this is due to following code:

   Py::Long k( s.ptr() );
            throw TestError( "convert a Py::String to an Py::Long must not succeed" );
        }
        catch( Py::TypeError &e )
        {
            std::cout << "PASSED: Correctly caught " << Py::type(e) << std::endl;

and it asserts here:

   │508     #ifdef Py_DEBUG                                                                                            │
   │509         /* PyObject_Str() must not be called with an exception set,                                            │
   │510            because it may clear it (directly or indirectly) and so the                                         │
   │511            caller looses its exception */                                                                      │
  >│512         assert(!PyErr_Occurred());                                                                             │
   │513     #endif   

the reason is the example catches an exception but does not reset the exception within python. When it calls PyObject_Str on the exception it asserts.

[Barry Alan Scott]

Is the patch I need to call e.clear(); before calling
Py:type(e) etc?

Py::Long k( s.ptr() );
throw TestError( "convert a Py::String to an Py::Long must not succeed" );
}
catch( Py::TypeError &e )
{
e.clear(); // Fix crash in Py_DEBUG mode
std::cout << "PASSED: Correctly caught " << Py::type(e) << std::endl;

[Julian Taylor]

its required in any exception handler that calls a python method that can set an error.
Is there a better way to do this? within c++ one already has caught the exception and thus one hasn't lost it as python assumes.
E.g. clearing the error on construction of all exceptions.

[Julian Taylor]

It might also be worth asking python devs to revert this, I assume it could affect all high level wrappers of the python capi.

[Barry Alan Scott]

From the comment the assert is simply telling you that the code will not do what you think. It seems that the e.clear() will be required.

I don't have adebug build of 3.4 on hand. Could you try the patch and if it works I'll commit the change and release a updated pycxx kit. THere are number of other minor fixes that could do with a release.

[Julian Taylor]

it will certainly work, but I don't think its the right solution.

[Julian Taylor]

on the other hand it seems to be already documented:
http://cxx.sourceforge.net/PyCXX-Python3.html#h2_exceptions_catching
so its probably fine.

tested the patch works with py3.4beta3

[Barry Alan Scott]

Thanks I'll apply the patch.

[Barry Alan Scott]

Commited as r295