使用Python3 C API覆盖对象方法

Question

I'm working on porting the core functionality of a project from python to C, but want to retain the ability to call into modular Python components. So I'm embedding the python interpreter in my project. I've gotten to the point where calls can be made on both the C, and Python side to the other end, and shared data is converted and correctly handled.

However, I seem unable to override methods in my defined Python classes. Rather, there's no errors, however self is not set for my PyCFunction .

Basic example code is like:

component.py

class Component(object):
  def my_function(self):
    print("orig function")


class CompObj(Component):
  def test(self):
    self.my_function()

example.c

#include <Python.h>

static PyObject *my_function_override(PyObject *self, PyObject *args) {
  printf("Override method %p\n", self);
  return Py_None;
}

PyMethodDef override_method = {
  "my_function",
  (PyCFunction)my_function_override,
  METH_VARARGS,
  NULL
};

int main () {
  wchar_t *program;
  PyObject *sys_path,  *module_path;
  PyObject *pModule, *pCompObj, *pCompObjInst;
  PyObject *cfunc, *cmeth;

  program = Py_DecodeLocale("exmaple", NULL);
  Py_SetProgramName(program);
  Py_InitializeEx(0);

  sys_path = PySys_GetObject("path");
  module_path = PyUnicode_FromString("/Users/dwalker/ex");
  PyList_Append(sys_path, module_path);
  Py_DECREF(module_path);

  if ((pModule = PyImport_ImportModule("component")) == NULL) {
    printf("No Module\n");
    return -1;
  }

  pCompObj = PyObject_GetAttrString(pModule, "CompObj");
  pCompObjInst = PyObject_CallFunction(pCompObj, NULL);

  cfunc = PyCFunction_New(&override_method, NULL);
  cmeth = PyMethod_New(cfunc, pCompObjInst);

  PyObject_SetAttrString(pCompObjInst, "my_function", cmeth);
  PyObject_CallMethod(pCompObjInst, "test", NULL);

  Py_Finalize();
  PyMem_RawFree(program);

  return 0;
}

CMakeLists.txt

set(SRC_CORE
  example.c
)

add_executable(example ${SRC_CORE})

set_property(TARGET example PROPERTY C_STANDARD 90)

find_package(PythonInterp)
find_package(PythonLibs)
include_directories(${PYTHON_INCLUDE_PATH})
target_link_libraries(example ${PYTHON_LIBRARIES})

So, here after I build and run this I get the output:

Override method 0x0

The override method is being called, but the instance is not being passed as the first self argument. I assume I'm doing something wrong binding the method to the instance. For what it's worth, I've also tried similar execution paths, by setting the method via PyInstanceMethod_New to pCompObj and not the instance. But no matter what, I'm unsure how to get the self value correctly passed.

Answer 1

After more playing, and finally looking at the CPython source ( https://github.com/python/cpython/blob/master/Objects/methodobject.c ) it became obvious that using PyMethod_New(PyObject *func, PyObject *self) and setting this return was not correct. The PyCFunction call accepts two arguments, the latter being the instance to use for self. So in my example, changed to the following we have success:

pCompObj = PyObject_GetAttrString(pModule, "CompObj");
pCompObjInst = PyObject_CallFunction(pCompObj, NULL);

cfunc = PyCFunction_New(&override_method, pCompObjInst);

PyObject_SetAttrString(pCompObjInst, "my_function", cfunc);
Py_DECREF(cfunc);

So this seems to be what I'm intending to do, however, anyone much more versed in the Python C API can follow up with a "more correct" answer.