[Swig-devel] [ swig-Bugs-1821415 ] Python directors & dirvtable bug

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Bugs item #1821415, was opened at 2007-10-28 13:14
Message generated for change (Settings changed) made by olly
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>Category: python
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: nitro (nitrogenycs)
Assigned to: Nobody/Anonymous (nobody)
Summary: Python directors & dirvtable bug

Initial Comment:
The following text is almost identical to this one:

http://thread.gmane.org/gmane.comp.programming.swig/11560

-------------------------------------


Hello guys,

there seems to be a bug in swig regarding python directors. Attached is a  
minimal interface file which reproduces the problem.

Swig generates code like this:

#if defined(SWIG_PYTHON_DIRECTOR_VTABLE)
   const size_t swig_method_index = 0;
   const char * const swig_method_name = "my_method";
   PyObject* method = swig_get_method(swig_method_index, swig_method_name);
   swig::PyObject_var result = PyObject_CallFunctionObjArgs(method  
,(PyObject *)obj0, NULL);
#else
   swig::PyObject_var swig_method_name = PyString_FromString((char  
*)"Render");
   swig::PyObject_var result = PyObject_CallMethodObjArgs(swig_get_self(),  
(PyObject *) swig_method_name ,(PyObject *)obj0, NULL);
#endif

In my case SWIG_PYTHON_DIRECTOR_VTABLE is defined, so only the first  
branch is executed. This includes the call to swig_get_method. This  
function is generated for each python director class. It tries to retrieve  
the python method called "my_method" from the current object. If this  
fails, because the method wasn't defined in python it triggers this line  
within swig_get_method:

Swig::DirectorMethodException::raise(msg.c_str());

Unfortunately I don't think there is any way to capture this exception or  
to override it somehow. My program just crashes :(. I am already using  
%feature("director:except") and %exception. However, these are useless as  
they don't enclose the position where the DirectorMethodException is  
raised.
Notice that if SWIG_PYTHON_DIRECTOR_VTABLE is not defined (alas, running  
swig without -dirvtable and/or without -O) the swig code will rise a  
DirectorMethodException a bit later. By using %feature("director:except")  
I can override this behaviour though. Overriding just doesn't work in the  
case where SWIG_PYTHON_DIRECTOR_VTABLE is defined.
One solution I can see is to make swig_get_method return 0 instead of  
throwing an exception. The calling code would look like this:

#if defined(SWIG_PYTHON_DIRECTOR_VTABLE)
   const size_t swig_method_index = 0;
   const char * const swig_method_name = "my_method";
   PyObject* method = swig_get_method(swig_method_index, swig_method_name);
   swig::PyObject_var result = method ? PyObject_CallFunctionObjArgs(method  
,(PyObject *)obj0, NULL) : 0;
#else
   swig::PyObject_var swig_method_name = PyString_FromString((char  
*)"Render");
   swig::PyObject_var result = PyObject_CallMethodObjArgs(swig_get_self(),  
(PyObject *) swig_method_name ,(PyObject *)obj0, NULL);
#endif

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You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=101645&aid=1821415&group_id=1645