Creating a python object in C++ and calling its method

Question

In the following document about python embedding, it is well described how to embed python methods in "C" code. https://docs.python.org/3/extending/embedding.html

I tested the above code, it works well with the GCC g++ compiler as well.

But the above shows examples of calling the global methods not the class methods.

Could anyone show an example about how to create a Python object and call its method from C++?

Answer 1

By doing some investigation, I found that this can be done using the following fourAPIs in series.

• PyModule_GetDict ; Gets items belonging to the python module. *

• PyDict_GetItemString ; Gets the item corresponding to Python class name.

• PyObject_CallObject; Creates the Python object. *
• PyObject_CallMethod; Class a method of the object.

The following is the sample code that I created even though it still needs to be improved.

// Refer to the following website for more information about embedding the
// Python code in C++.
// https://docs.python.org/2/extending/embedding.html
int main() {
  PyObject *module_name, *module, *dict, *python_class, *object;

  // Initializes the Python interpreter
  Py_Initialize();

  module_name = PyString_FromString(
      "work.embedding_python_in_cpp.example.adder");

  // Load the module object
  module = PyImport_Import(module_name);
  if (module == nullptr) {
    PyErr_Print();
    std::cerr << "Fails to import the module.\n";
    return 1;
  }
  Py_DECREF(module_name);

  // dict is a borrowed reference.
  dict = PyModule_GetDict(module);
  if (dict == nullptr) {
    PyErr_Print();
    std::cerr << "Fails to get the dictionary.\n";
    return 1;
  }
  Py_DECREF(module);

  // Builds the name of a callable class
  python_class = PyDict_GetItemString(dict, "Adder");
  if (python_class == nullptr) {
    PyErr_Print();
    std::cerr << "Fails to get the Python class.\n";
    return 1;
  }
  Py_DECREF(dict);

  // Creates an instance of the class
  if (PyCallable_Check(python_class)) {
    object = PyObject_CallObject(python_class, nullptr);
    Py_DECREF(python_class);
  } else {
    std::cout << "Cannot instantiate the Python class" << std::endl;
    Py_DECREF(python_class);
    return 1;
  }

  int sum = 0;
  int x;

  for (size_t i = 0; i < 5; i++) {
    x = rand() % 100;
    sum += x;
    PyObject *value = PyObject_CallMethod(object, "add", "(i)", x); 
    if (value)
      Py_DECREF(value);
    else
      PyErr_Print();
  }
  PyObject_CallMethod(object, "printSum", nullptr);
  std::cout << "the sum via C++ is " << sum << std::endl;

  std::getchar();
  Py_Finalize();

  return (0);
}