Problems calling Python from C++

Question

test.py

def add(a,b):
 """  """
 print a,b,a+b
 return a+b

c program

#include <python.h>
int _tmain(int argc, _TCHAR* argv[])
{
 try
 {
  PyObject *pName,*pModule,*pDict,*pFunc,*pArgs1,*pArgs2,*pOutput;

  Py_Initialize();
  if(!Py_IsInitialized())
   return -1;
  pModule=PyImport_ImportModule("test");

  pDict=PyModule_GetDict(pModule);

  pFunc=PyDict_GetItemString(pDict,"add");
  pArgs1=Py_BuildValue("ii", 1,2); 
  //pArgs2=Py_BuildValue("i", 2); 

  pOutput=PyEval_CallObject(pFunc,pArgs1);

  int c=0;
  PyArg_Parse(pOutput, "d", &c);
  cout<<c;

  //PyRun_SimpleString("");

  Py_Finalize();
 }
 catch(exception* ex)
 {
  cout<<ex->what();
 }
 char c;
 cin>>c;
 return 0;
}

Console print nothing and closed.

What's wrong?

Thanks!

Maybe not in general, but it is for me: I know C/Python, but I haven't touched C++ in years. Tagging your questions correctly will net you better answers :) — detly
– detly, Commented Jun 23, 2010 at 15:01

Jason R. Coombs · Accepted Answer · 2010-06-23 12:12:27Z

3

Last I checked, C doesn't have exceptions. Surely, you're not going to get any exceptions thrown by calls to the Python lib.

First, since you're using C++, you may need to include the Python lib with an extern declaration.

extern "C" {
    #include "python.h"
}

Next, since you don't have exceptions in C calls, you should test the result of each call as you go along. This will help you better understand where it's failing.

Since you're not getting a segfault or anything, I suspect you're getting to

if(!Py_IsInitialized())
 return -1;

And exiting. Instead, you could print the return value so you know what's happening.

int is_init = Py_IsInitialized();
cout << "are we initialized? " << is_init;
if(!is_init)
    return -1;

If that doesn't demonstrate the trouble, then add additional cout statements throughout your code to trace where the problem is occurring... or better yet, use a debugger and step through the code as it runs. Surely you'll find what's going wrong.

answered Jun 23, 2010 at 12:12

Jason R. Coombs

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4 Comments

Begtostudy Over a year ago

pArgs1=Py_BuildValue("ii", 1,2); pOutput=PyEval_CallObject(pFunc,pArgs1); Is it right?

Jason R. Coombs Over a year ago

I think you need pArgs1=Py_BuildValue("(ii)", 1, 2); Also, you need to Py_DECREF(pArgs1); after you're done with it (after PyEval_CallObject).

Begtostudy Over a year ago

pModule=PyImport_ImportModule("test"); this line return null I put the py with the output exe, is thar right? Why module import error?

Jason R. Coombs Over a year ago

It's hard to say. One thing you might try is set an environment variable PYTHONPATH=/mylibs (or whatever) and then put test.py in that directory. Also, make sure your test.py is valid for the Python version you're using. If you're compiling against Python 2.x, you should be fine, otherwise, use 'print(a,b,a+b)'

Begtostudy · Accepted Answer · 2010-06-28 08:38:52Z

0

I found it contains some chinese words in first line.

#XXX

And, it also didn't work in pythonwin. Said something wrong.

So, I deleted them, and it's OK!

edited Jun 28, 2010 at 8:38

answered Jun 25, 2010 at 15:21

Begtostudy

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