Question

I’ve a simplest test case here:

%module test
%{

static char* MyExceptionName = "_test.MyException";
static PyObject* MyException = NULL;

%}

%inline %{

static PyObject* Foo()
{
    PyErr_SetNone(MyException);
    return NULL;
}

%}

%init
{
    MyException = PyErr_NewException(MyExceptionName, NULL, NULL);
}

Here’s the setup.py script:

from distutils.core import setup, Extension
setup(name="test", version="1.0",
    ext_modules = [Extension("_test", ["test_wrap.c"])])

When I build it and test it as follows, I get:

 swig -python -threads test.i
 python_d -c "import test; test.Foo()"
 Fatal Python error: PyThreadState_Get: no current thread

The traceback I got was

python27_d.dll!Py_FatalError(const char * msg=0x000000001e355a00)  Line 1677    C
python27_d.dll!PyThreadState_Get()  Line 330    C
python27_d.dll!PyErr_Restore(_object * type=0x00000000020d50b8, _object * value=0x0000000000000000, _object * traceback=0x0000000000000000)  Line 27 + 0x5 bytes    C
python27_d.dll!PyErr_SetObject(_object * exception=0x00000000020d50b8, _object * value=0x0000000000000000)  Line 58 C
python27_d.dll!PyErr_SetNone(_object * exception=0x00000000020d50b8)  Line 64   C
_test_d.pyd!Foo()  Line 2976    C

Environment:

• Win 7 64 bit,
• Python 2.7.3 (default, Aug 15 2012, 18:18:52) [MSC v.1500 64 bit (AMD64)] on win32
• Swig 2.0.7

Answer 1

The reason for the error, as it turns out is because when -threads is enabled via

swig -threads -python test.i

We get something like this (excess code has been redacted):

PyObject *_wrap_Foo(PyObject *SWIGUNUSEDPARM(self), PyObject *args) {
  PyObject *resultobj = 0;
  PyObject *result = 0 ;

  if (!PyArg_ParseTuple(args,(char *)":Foo")) SWIG_fail;
  {
    SWIG_PYTHON_THREAD_BEGIN_ALLOW;
    result = (PyObject *)Foo();
    SWIG_PYTHON_THREAD_END_ALLOW;
  }
  resultobj = result;
  return resultobj;
fail:
  return NULL;
}

static PyObject* Foo()
{
    PyErr_SetNone(MyException);
    return NULL;
}

See, when Foo() is called, the global interpreter lock has already been released. Foo() should no longer make any Python API calls.

The solution is to use SWIG_Python_SetErrorObj, which grabs the Global Interpreter Lock before calling Python C API.

static PyObject* Foo()
{
    SWIG_Python_SetErrorObj(MyException, Py_None);
    return NULL;
}

Another method is to use SWIG_PYTHON_THREAD_BEGIN_BLOCK; and SWIG_PYTHON_THREAD_END_BLOCK;

static PyObject* Foo()
{
    SWIG_PYTHON_THREAD_BEGIN_BLOCK; 

    PyErr_SetNone(MyException);

    SWIG_PYTHON_THREAD_END_BLOCK;
    return NULL;
}