C++ Embeded Python :pass Tuple declared in C++ to a method of a python class

Question

it's ok to pass a tuple as argument of a method, but as soon as I want to pass the tuple to a method of a class it doesn't work(I get a Run Failed at the row "ret = PyEval_CallObject(method,args);" thanks a lot if someone knows why it doesn't work

the following code used is:

enter code Python Code:

class cVector:
  def __init__(self,msg):
    self.value = msg
  def ComputeNorm(self,vecData):
    #don't use vecData for instance
    result = 12.
    return(result)

enter C++ Code

PyObject *ret, *mymod, *pclass, *method, *args, *object;
float retValue;

Py_Initialize();
PySys_SetPath(".");

// Module
mymod = PyImport_ImportModule("mModule8");
if (mymod == NULL){
  cout << "Can't Open a module:\n" ;
  Py_DECREF(mymod);
}

// Class
pclass = PyObject_GetAttrString(mymod, "cVector");
if (pclass == NULL) {
  Py_DECREF(pclass);
  cout << "Can't find class\n";
}

// Parameters/Values
args = Py_BuildValue("(f)", 100.0);
if (args == NULL) {
  Py_DECREF(args);
  cout << "Can't build argument list for class instance\n";
}

// Object with parameter/value
object = PyEval_CallObject(pclass, args);
if (object == NULL) {
  Py_DECREF(object);
  cout << "Can't create object instance:\n";
}

// Decrement the argument counter as we'll be using this again 
Py_DECREF(args);

// Get the object method - note we use the object as the object
// from which we access the attribute by name, not the class 
method = PyObject_GetAttrString(object, "ComputeNorm");
if (method == NULL) {
  Py_DECREF(method);
  cout << "Can't find method\n";
}

// Decrement the counter for our object, since we now just need
// the method reference 
Py_DECREF(object);

// Build our argument list - an empty tuple because there aren't
// any arguments 

cout << "Prepare the Tuple:\n" ;
// WE pass a tuple
args = PyTuple_New( 3 );
if (args == NULL) {
  Py_DECREF(args);
  cout << "Can't build argument list for method call\n";
}

PyObject  *py_argument;
// 1st argument 
py_argument = PyFloat_FromDouble(5.);
PyTuple_SetItem(args, 0, py_argument);

// 2nd argument 
py_argument = PyFloat_FromDouble(10.);
PyTuple_SetItem(args, 1, py_argument);

// 3nd argument 
py_argument = PyFloat_FromDouble(15.);
PyTuple_SetItem(args, 2, py_argument);

cout << "Before the Exec:\n" ;
// Call our object method with arguments 
ret = PyEval_CallObject(method,args);
//ret = PyObject_CallObject(method,args);
if (ret == NULL) {
  Py_DECREF(ret);
  cout << "Couldn't call method\n";
}


// Convert the return value back into a C variable and display it 
PyArg_Parse(ret, "f", &retValue);
printf("RetValue: %f\n", retValue);
// Kill the remaining objects we don't need 
Py_DECREF(method);
Py_DECREF(ret);
// Close off the interpreter and terminate 
Py_Finalize();

"it doesn't work" is really uninformative. What's wrong? Also, how do you initialize args? Can't check this right now, but remember that tuples are immutable and you can only PyTuple_SetItem as many items as you declared in PyTuple_New. Also, have you tried, maybe, PyObject_CallFunction, which supports a nice, PyObject_BuildValue-style of building a tuple right in place? — maligree
– maligree, Commented Jan 9, 2012 at 12:01

yak · Accepted Answer · 2012-01-09 12:39:37Z

2

You don't show how you obtain method. You have to get it from an instance for this to work (here I assume that inst is a PyObject* pointing to an instance of cVector class):

PyObject *method = PyObject_GetAttrString(inst, "ComputeNorm");

Always check for errors:

if (method == NULL)
    return NULL;

(or do other appropriate thing, depending on the context)

Then, your code can be greatly shortened:

PyObject *args = Py_BuildValue("(ddd)", 5.0, 10.0, 15.0);

(this creates a tuple with three Python floats made from C doubles)

or even combined with the call:

PyObject *ret = PyObject_CallFunction(method, "(ddd)", 5.0, 10.0, 15.0);

You call even combine everything in one call:

PyObject *ret = PyObject_CallMethod(inst, "ComputeNorm",
                                    "(ddd)", 5.0, 10.0, 15.0);

Again, remeber to check for errors:

if (ret == NULL)
   return NULL;

And always decref all the objects you create and don't need anymore (otherwise you will be leaking memory):

Py_DECREF(ret);

(assuming you've used the PyObject_CallMethod, otherwise you might have to decref args and method as well)

edited Jan 9, 2012 at 12:39

answered Jan 9, 2012 at 12:16

yak

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6 Comments

pasparis Over a year ago

thank you for your short reply Sorry I didn't put the complete code (method,class initialization) as I think it works,the code is the following (I want to "dynamically' initialize the tuple in c++

pasparis Over a year ago

thanks again for your fast reply, I can't put the code online before 6 hours time (constraint from stackoverflow for a first time subscriber)I 'll do it soon

pasparis Over a year ago

your solution doesn't match with my need which is to feed a tuple with many elements from database.the piece of code works in this case,but it works with a direct method in a module file and not with a method in a class.

yak Over a year ago

You'll have to be more specific than "doesn't work" (post all relevant code, say what happens, say what did you expect, post complete error messages, etc.).

pasparis Over a year ago

thanks Yak, I will do what you day after the 6 hours required by stackoverflow (a new subscriver can't post a new message immediatly he has to wait 6 hours;

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