how to deal with the PyObject* from C++ in Python

Question

I create DLL wrote in C++ , the exporting function returns PyObject * .Then I use ctypes to import the DLL in Python . Now , how can I get the real PyObject ??

here's some part of c++ code:

PyObject* _stdcall getList(){
 PyObject * PList = NULL;
 PyObject * PItem = NULL;
 PList = PyList_New(10);
 vector <int> intVector;
 int i;
 for(int i=0;i<10;i++){
 intVector.push_back(i);
 }
 for(vector<int>::const_iterator it=intVector.begin();it<intVector.end();it++){
 PItem = Py_BuildValue("i", &it);
 PyList_Append(PList, PItem);
 }
 return PList;
}

and some python code :

dll = ctypes.windll.LoadLibrary(DllPath)
PList = dll.getList()

*I wanna get the real python list containing 1,2,3,4...10 ? * Am I clear ?? Thanks advance

• Ignacio Vazquez-Abrams: Why not finish writing the module instead of using ctypes?
• Aha_1024: because I don't know what to do next...at that time
• Ignacio Vazquez-Abrams: docs.python.org/2/extending/extending.html

Answer 1

You have a number of issues of your code, some modifications:

#include <Python.h>
#include <vector>
extern "C" PyObject* _stdcall getList(){
 PyObject *PList = PyList_New(0);
 std::vector <int> intVector;
 std::vector<int>::const_iterator it;
 for(int i = 0 ; i < 10 ; i++){
 intVector.push_back(i);
 }
 for(it = intVector.begin(); it != intVector.end() ; it++ ){
 PyList_Append(PList, Py_BuildValue("i", *it));
 }
 return PList;
}

compile it:

> g++ -Wall -shared lib.cpp -I \Python27\include -L \Python27\libs -lpython27 -o lib.dll -Wl,--add-stdcall-alias

now you can load it as any function and set the getList return type to py_object as:

import ctypes
lib = ctypes.WinDLL('lib.dll')
getList = lib.getList
getList.argtypes = None
getList.restype = ctypes.py_object
getList()

test it:

>>> import ctypes
>>>
>>> lib = ctypes.WinDLL('lib.dll')
>>>
>>> getList = lib.getList
>>> getList.argtypes = None
>>> getList.restype = ctypes.py_object
>>> getList()
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>>
>>>

• Aha_1024: thanks advance! learn from you , I modify my code , but finally return a python list like "[ " .btw,I use vs2010 to create the lib.dll. Does it metter??
• Aha_1024: I found a solution to my problem. I pass a pyobject list as parameter to c++ function, in the function I add items to the list. Finally ,it works
• Eryk Sun: @Huhu: Hold the GIL for this. ctypes defaults to releasing the GIL. Use a function pointer type with the flag ctypes._FUNCFLAG_PYTHONAPI, or a subclass of the CDLL class with _func_flags_ = ctypes._FUNCFLAG_STDCALL | ctypes._FUNCFLAG_PYTHONAPI.

Answer 2

With Visual Studio, and Python 64 bits:
1- Create an empty Win32 Project (DLL Type)
2- Right Click on your Solution Project -> Configuration Manager
3- Active Solution configuration (Release)
4- Active Solution Platform -> New, then on the bottom Dropdown list, select x64 -> OK
5- In the Source Files Folder, add an empty C++ file
6- Put your C++ code (One modification for getList to be recognized)

#include <Python.h>
#include <vector>
extern "C" __declspec(dllexport) PyObject* _stdcall getList();
PyObject* _stdcall getList(){
 PyObject *PList = PyList_New(0);
 std::vector <int> intVector;
 std::vector<int>::const_iterator it;
 for (int i = 0; i < 10; i++){
 intVector.push_back(i);
 }
 for (it = intVector.begin(); it != intVector.end(); it++){
 PyList_Append(PList, Py_BuildValue("i", *it));
 }
 return PList;
}

Answer 3

I'm not exactly clear what you are asking. But I suppose you mean to ask what you can do now with your DLL.

1. Well, in order to use it appropriately, you'll have to build a special DLL which directly can be imported as a module in Python. In order to determine what to do in order to use this, it is best you have a look for other modules, how they do it. E. g. MySQLdb could be a candidate.

   In short, you have this "wrapper" DLL call your function.

2. But if I have a second look at your question now, I see that you are trying to load your DLL via ctypes. This is viable as well, maybe even better, and you'll have to use the ctypes.py_object data type.

• Aha_1024: I use "ctypes.py_object" and it works. excited~ Thank you very much