How to call a python function in C++

[Joseph A Knapka]

clio wrote:
> 
> Hi,
> Here is a very confusing problem. I am trying to call a python function
> in C. I build a .cxx which have following functions:
> 
> ......
> static PyObject *my_callback=NULL;
> 
> static PyObject * my_set_callback(PyObject * dummy,PyObject * args)
>
>   {
>             PyObject *result = NULL;
>             PyObject *temp;
> 
>             if (PyArg_ParseTuple(args, "O:set_callback", &temp)) {
>                 if (!PyCallable_Check(temp)) {
>                     PyErr_SetString(PyExc_TypeError, "parameter must be
> callable");
>                     return NULL;
>                 }
>                 Py_XINCREF(temp);         /* Add a reference to new
> callback */
>                 Py_XDECREF(my_callback);  /* Dispose of previous callback */
>                 my_callback = temp;       /* Remember new callback */
>                 /* Boilerplate to return "None" */
>                 Py_INCREF(Py_None);
>                 result = Py_None;
>             }
>             return result;
>         }
> ..........
> Acctually, this code is from online manual. Then in another c++ program
> I have this:
> 
> extern PyObject *my_callback;

This will not work because my_callback is declared static
and therefor is visible only within its source file. Try

PyObject *my_callback;

instead.

> ......
> PyObject * arglist;
> arglist = Py_BuildValue("(s1000)", buf);
> PyObject *result;
> 
> result = PyEval_CallObject(my_callback, arglist);
> Py_DECREF(arglist);
> 
> .......
> 
> I compiled them to seprate object file and link them to be provapimodule.so.
> 
> I got no error in compiling, but when I load the provapi in python,
> there is an error:
> 
>  >ImportError: ./provapimodule.so: undefined symbol: my_callback.

See my comment above. There is essentially no link step at build time
for a Linux shared library; the Linux dynamic loader attempts to resolve
linkage at load time, so that's why you see the error here rather than
at build time.

> Then I try to remove "static" in defination of my_callback. This time, I
> can load the module without error, and I can access provapi.my_callback
> too. But I met this:
> 
>  >>> provapi.my_set_callback(binstr,happy)
> Traceback (innermost last):
>    File "<stdin>", line 1, in ?
> AttributeError: my_set_callback
> 
> I do not know what does this mean?

You -have- initialized the module method table using
Py_InitModule(), haven't you? If not, that's the
problem.

> I am using g++ (gcc version 2.96 20000731) and Linux 7.1. Python 1.5.2.
> 
> Anyone can help me, thank you!
> 
> clio

HTH,

-- Joe
# "You know how many remote castles there are along the
#  gorges? You can't MOVE for remote castles!" - Lu Tze re. Uberwald
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