Mailing List Archive

Albert Hofkamp <hat@se-46.wpa.wtb.tue.nl> wrote:
: Hello all,

: I have been experimenting with c++ and Python, and am working on
: attaching generated c++ module-code to python.

: In the EXT document, it is explained how to parse arguments from python
: in c code using PyTuple_ParseArgs(). This is really easy to use.

I think you mean PyArg_ParseTuple().

: However, the same kind of parsing must occur after a call from c to a
: python function. The return value from that function is again a Python
: object, and needs to be decoded before it can be used in c.
: Unfortunately, the returned value is not a tuple, so PyTuple_ParseArgs()
: cannot be used. A shortcut would be to wrap the result in a tuple with 1
: argument, and then call the decoding function, but I consider that a hack.

: Wouldn't it be possible to create a PyTuple_ParseArgs()-like function to
: parse return results from python functions ?

Gee... that is what happens when you take something out of the doc.

There is indeed a function that you describe. It is not used often,
but "in the old days" it was used instead of PyArg_ParseTuple(). The
function is called PyArg_Parse() and works identically to the function
PyArg_ParseTuple(), except that it does not expect a tuple, it expects
any Python object.

PyObject *function;
PyObject *result, *args;
char *street, *town, *city;
int zipcode;

args = Py_BuildValue("ss", "name", "Arcege");
/* returns a list of four elements */
result = PyObject_CallObject(function, args);
Py_DECREF(args);
if (!PyArg_Parse(result, "[sssi]", &street, &town, &city, &zipcode))
return NULL;

Often, it is better to determine the type of the return value in the C
code manually - making it more robust and extensible.

result = PyObject_CallObject(function, args);
if (result == Py_None) {
printf("No address found\n");
Py_DECREF(Py_None);
} else if (!PyList_Check(result)) {
PyErr_SetString(PyExc_TypeError, "expecting a list (or None)")
return NULL;
}

A good idea is to go thru the Python sources and see how PyArg_Parse
is still being used.

-Arcege