[Python-Dev] Strange behaviour with PyEval_EvalCode

Joe Eagar joeedh at gmail.com
Sun May 13 21:59:06 CEST 2007

Hi I'm getting extremely odd behavior.  First of all, why isn't
PyEval_EvalCode documented  anywhere?  Anyway, I'm working on blender's
python integration (it embeds python, as opposed to python embedding
it).  I have a function that executes a string buffer of python code,
fetches a function from its global dictionary then calls it.

When the function code returns a local variable, PyObject_Call() appears
to be returning garbage.  Strangely this is only happening with internal 
blender types, yet try however I might I can't find any refcounting 
errors to account for this.  The initial implementation used the same
dictionary for the global and local dicts.  I tried using separate
dicts, but then the function wasn't being called at all (or at least I
tested it by putting a "print "bleh"" in there, and it didn't work). 
Also, when I printed the refcount of the garbage data, it was garbage as 
well (so the entire piece of memory is bad, not just the data portion).

I've tested with both python 2.4 and 2.5.   Mostly with 2.4.  This bug
may be cropping up in other experimental blender python code  as well.

Here's the code in the string buffer:
from Blender import *
from Blender.Mathutils import *
print "d"
def doConstraint(inmat, tarmat, prop):
    a = Matrix()
    a = a * TranslationMatrix(Vector(0, 0, 0))
    print "t"
    a = tarmat
    return inmat

print doConstraint(Matrix(), Matrix(), 0)

Here's the code that executes the string buffer:

PyObject *RunPython2( Text * text, PyObject * globaldict, PyObject
*localdict )
    char *buf = NULL;

/* The script text is compiled to Python bytecode and saved at
* to speed-up execution if the user executes the script multiple times */

    if( !text->compiled ) {    // if it wasn't already compiled, do it now
        buf = txt_to_buf( text );

        text->compiled =
            Py_CompileString( buf, GetName( text ),
                      Py_file_input );

        MEM_freeN( buf );

        if( PyErr_Occurred(  ) ) {
            BPY_free_compiled_text( text );
            return NULL;

    return PyEval_EvalCode( text->compiled, globaldict, localdict );

. . .and heres the (rather long, and somewhat in a debugging state)
function that calls the function in the script's global dictionary:

void BPY_pyconstraint_eval(bPythonConstraint *con, float obmat[][4],
short ownertype, void *ownerdata, float targetmat[][4])
    PyObject *srcmat, *tarmat, *idprop;
    PyObject *globals, *locals;
    PyObject *gkey, *gval;
    PyObject *retval;
    MatrixObject *retmat;
    Py_ssize_t ppos = 0;
    int row, col;

    if ( !con->text ) return;

    globals = CreateGlobalDictionary();

    srcmat = newMatrixObject( (float*)obmat, 4, 4, Py_NEW );
    tarmat = newMatrixObject( (float*)targetmat, 4, 4, Py_NEW );
    idprop = BPy_Wrap_IDProperty( NULL, &con->prop, NULL);

/*  since I can't remember what the armature weakrefs do, I'll just
leave this here
    commented out.  Since this function was based on pydrivers.
    if( !setup_armature_weakrefs()){
        fprintf( stderr, "Oops - weakref dict setup\n");
        return result;
    retval = RunPython2( con->text, globals, globals);

    if (retval) {Py_XDECREF( retval );}

    if ( retval == NULL ) {
        ReleaseGlobalDictionary( globals );

        /*free temp objects*/
        Py_XDECREF( idprop );
        Py_XDECREF( srcmat );
        Py_XDECREF( tarmat );

    /*Now for the fun part! Try and find the functions we need.*/
    while ( PyDict_Next(globals, &ppos, &gkey, &gval) ) {
        if ( PyString_Check(gkey) && strcmp(PyString_AsString(gkey),
"doConstraint")==0 ) {
            if (PyFunction_Check(gval) ) {
                retval = PyObject_CallObject(gval, Py_BuildValue("OOO",
srcmat, tarmat, idprop));
                Py_XDECREF( retval );
            } else {
                printf("ERROR: doConstraint is supposed to be a

    if (!retval) {
        /*free temp objects*/
        ReleaseGlobalDictionary( globals );

        Py_XDECREF( idprop );
        Py_XDECREF( srcmat );
        Py_XDECREF( tarmat );

    if (!PyObject_TypeCheck(retval, &matrix_Type)) {
        printf("Error in pyconstraint: Wrong return type for a
        ReleaseGlobalDictionary( globals );

        Py_XDECREF( idprop );
        Py_XDECREF( srcmat );
        Py_XDECREF( tarmat );
        Py_XDECREF( retval );

    retmat = (MatrixObject*) retval;
    if (retmat->rowSize != 4 || retmat->colSize != 4) {
        printf("Error in pyconstraint: Matrix is the wrong size!\n");
        ReleaseGlobalDictionary( globals );

        Py_XDECREF( idprop );
        Py_XDECREF( srcmat );
        Py_XDECREF( tarmat );
        Py_XDECREF( retval );

    //this is the reverse of code taken from newMatrix().
    for(row = 0; row < 4; row++) {
        for(col = 0; col < 4; col++) {
            if (retmat->wrapped) obmat[row][col] =
retmat->data.blend_data[row*4+col]; //[row][col];
            else obmat[row][col] = retmat->data.py_data[row*4+col];


    /*clear globals*/
    //ReleaseGlobalDictionary( globals );

    /*free temp objects*/
    //Py_XDECREF( idprop );
    //Py_XDECREF( srcmat );
    //Py_XDECREF( tarmat );
    //Py_XDECREF( retval );


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