PyObject_CallMethod Returns Null, Simple Example Provided

Question

I am a beginner Python programmer, working on embedding some Python code into C++ DLL. Could not make PyObject_CallMethod work, it returned Null every time I called the Python 3.6 class method. Created a small example to illustrate the issue, please see the comments in the code. Would appreciate some guidance on what I am doing wrong.

    #include "stdafx.h"

using namespace std;

int TestBasicStats()
{
    _putenv_s("PYTHONPATH", ".");

    Py_Initialize();

    PyObject* module = PyImport_ImportModule("BasicStats");
    assert(module != NULL); // Returned non-null object

    PyObject* MyPyClass = PyObject_GetAttrString(module, "BasicStats");
    assert(MyPyClass != NULL);// Returned non-null object

    PyObject* myClassInstance = PyInstanceMethod_New(MyPyClass);
    assert(myClassInstance != NULL);// Returned non-null object

    PyObject* result = PyObject_CallMethod(myClassInstance, "AddItem", "(i)", 1);
    assert(result != NULL); // Failed - returned NULL

    result = PyObject_CallMethod(myClassInstance, "AddItem", "(i)", 2);
    assert(result != NULL); // Failed - returned NULL

    result = PyObject_CallMethod(myClassInstance, "get_Max", NULL);
    assert(result != NULL); // Failed - returned NULL

    printf("Min = %ld\n", PyLong_AsLong(result));
    printf("Min = %f\n", PyFloat_AsDouble(result));

    Py_Finalize();

    return 0;
}


int main()
{
    TestBasicStats();

    return 0;
}

The Python class called is as follows:

class BasicStats:
def __init__(self):
    self._m_min = self._m_max = self._m_sum = self._m_sumSqr = 0
    self._m_count = 0
    self.Reset()

def Reset(self):
    self._m_min = float("inf")
    self._m_max = float("-inf")
    self._m_sum = self._m_sumSqr = 0
    self._m_count = 0

def AddItem(self, value):
    self._m_count += 1
    if self._m_max < value:
        self._m_max = value

def get_Max(self):
    return self._m_max

Max = property(fget=get_Max)

md5i · Accepted Answer · 2018-04-09 04:23:58Z

2

The bit I don't understand here is the PyInstanceMethod_New() call. I really am not sure what it is doing at all, as I have never used this in any of my code. The PyObject you get from the PyObject_GetAttrString() call should be the type object, and as such, a callable, which can be called to create a new object of that type. So to create a BasicStats object, I would call the type object as a function:

PyObject* myClassInstance = PyObject_CallFunctionObjArgs(myPyClass, NULL);

answered Apr 9, 2018 at 4:23

md5i

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1 Comment

Stack Exchange Broke The Law Over a year ago

The documentation says that PyInstanceMethod_New creates a function - which explains why it doesn't work, it's not actually creating an instance.

J Evans · Accepted Answer · 2020-01-16 13:58:08Z

0

I believe that

PyObject* myClassInstance = PyInstanceMethod_New(MyPyClass);

really should be

// should call __init__. assumes __init__() has arity of 1
// otherwise you will have to marshall correct arguments
PyObject* myClassInstance = PyObject_CallObject(MyPyClass, 0);

answered Jan 16, 2020 at 13:58

J Evans

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