Retrieving exception value in C

[swapnil]

I am trying to retrieve the value of the exception (the message part)
raised in python, in C.

Running the below script,

import shutil
fd= open("testfile","w")
fd.write("some junk")
fd.close()
shutil.copy("testfile","testfile")

will generate an exception like this,

Traceback (most recent call last):
File "C:\Python26\lib\myscript.py", line 10, in <module>
shutil.copy("testfile","testfile")
File "C:\Python26\lib\shutil.py", line 88, in copy
copyfile(src, dst)
File "C:\Python26\lib\shutil.py", line 47, in copyfile
raise Error, "`%s` and `%s` are the same file" % (src, dst)
shutil.Error: `testfile` and `testfile` are the same file


But if I run (actually import) the script from within a C code
(embedding python in C), I am able to get the exception object but not
the value.
The code looks like,

int main()
{
PyObject *exc, *val, *tbk, *module, *name;
PyObject *exc_str;
Py_Initialize();
name = PyString_FromString("myscript");
module = PyImport_Import(name);
Py_DECREF(name);
if(!module)
{
printf("error in running script\n");
if( PyErr_Occurred())
{
if(PyErr_ExceptionMatches(PyExc_Exception))
{
printf("exception received in C\n");
}
PyErr_Fetch(&exc, &val, &tbk);
exc_str = PyObject_Str(exc);
printf("exception received: %s\n", PyString_AsString(exc_str));
printf("exception value: %s\n",PyString_AsString(val));
Py_DECREF(exc_str);
Py_DECREF(exc);
Py_DECREF(val);
Py_DECREF(tbk);
}
else{
printf("no exception received in C\n");
}

}
Py_XDECREF(module);
PyErr_Clear();
Py_Finalize();
return 0;
}

I get output,

error in running script
exception received in C
exception received: <class 'shutil.Error'>
exception value: (null)

While the last line should be,

exception value: `testfile` and `testfile` are the same file

Although I think its not required, FYI I'm running python 2.6.2 on
WinXP

 

[Gabriel Genellina]

I am trying to retrieve the value of the exception (the message part)
raised in python, in C.

But if I run (actually import) the script from within a C code
(embedding python in C), I am able to get the exception object but not
the value.
The code looks like,

int main()
{
PyObject *exc, *val, *tbk, *module, *name;
PyObject *exc_str;
Py_Initialize();
name = PyString_FromString("myscript");
module = PyImport_Import(name);
Py_DECREF(name);
if(!module)
{
printf("error in running script\n");
if( PyErr_Occurred())
{
if(PyErr_ExceptionMatches(PyExc_Exception))
{
printf("exception received in C\n");
}
PyErr_Fetch(&exc, &val, &tbk);
exc_str = PyObject_Str(exc);
printf("exception received: %s\n", PyString_AsString(exc_str));
printf("exception value: %s\n",PyString_AsString(val));
Py_DECREF(exc_str);
Py_DECREF(exc);
Py_DECREF(val);
Py_DECREF(tbk);
}
else{
printf("no exception received in C\n");
}

}
Py_XDECREF(module);
PyErr_Clear();
Py_Finalize();
return 0;
}

I get output,

error in running script
exception received in C
exception received: <class 'shutil.Error'>
exception value: (null)

While the last line should be,

exception value: `testfile` and `testfile` are the same file

PyErr_fetch retrieves the exception *type*, the exception *instance*, and
the traceback. So in your code exc is shutil.Error (the class) and val is
the specific instance raised.
The NULL comes from PyString_AsString(val) - it fails because val is not a
string object.
In Python code you'd get the message using str(val) - that would be
written PyObject_Str(val) in C.