Method without return value in python c extension module
Solution 1
Returning NULL to the python/c API indicates that an error has occurred. But since you didn't actually set an exception you get the error:
SystemError: error return without exception set
If you are trying to return None, use:
return Py_BuildValue("");
Solution 2
All python functions should return a PyObject, unless when they want to raise an exception, as explained: here http://docs.python.org/extending/extending.html#intermezzo-errors-and-exceptions
The error message you get SystemError: error return without exception set
, is trying to tell you that your function returned NULL (=error, raise an exception) but did not inform the python interpreter what exception you wanted to raise.
When you don't want to return a value from a python function you make it return None (which is same thing that happens if you in python code have a function that runs to the end or does a simple return without any value).
In the cpython api you do this by returning the Py_None object, and don't forget to increment its refcount. To help you not forgetting the refcount there is a macro to do it for you: http://docs.python.org/c-api/none.html#Py_RETURN_NONE.
So a function skeleton for a function returning nothing (=returning None) you look something like this:
static PyObject *
myfunction(PyObject *self, PyObject *args){
if (!PyArg_ParseTuple(args, "i", ...))
return NULL;
/* .... */
Py_RETURN_NONE;
}
Finally, for the record: there is a python module for doing the ioperm/outb calls already: http://pypi.python.org/pypi/portio
markmb
Updated on June 18, 2022Comments
-
markmb almost 2 years
I'm trying to create a script in python that sends data through a parallel port. I'm creating my own module in C language.
The problem is: when I try to execute my module, python crashes. No errors, no data, nothing. It simply closes.
This is my module:
#include <Python.h> #include <sys/io.h> #define BaseAddr 0x378 /*---------------------------------------------------------------------------------- Este es un módulo destinado a controlar el puerto paralelo. Probablemente tenga que ser ejecutado como administrador. Created by markmb ------------------------------------------------------------------------------------*/ static PyObject * paralelo(PyObject *self, PyObject *args){ int pin; ioperm(BaseAddr,3,1); if (!PyArg_ParseTuple(args, "i", &pin)) return NULL; outb(pin,BaseAddr); ioperm(BaseAddr,3,0); return 1 } PyMethodDef methods[] = { {"paralelo", paralelo, METH_VARARGS, "Sends data through a parallel port"}, {NULL, NULL, 0, NULL} }; PyMODINIT_FUNC initparalelo(void){ (void) Py_InitModule("paralelo", methods); }
(It works without all python mess) I compile it through distutils and then, in terminal (using xubuntu), I put:
import paralelo while True: paralelo.paralelo(255)
And here, it goes out of python, it puts "markmb@..."
Thanks in advance!
-
markmb over 12 yearsI used what the other comment says: return Py_None, and it works
-
Winston Ewert over 12 years@markmb, careful! If you use return Py_None, you need to increment the reference count or else you will run into trouble. You should either use what I have, or the Py_RETURN_NONE that the other answer has or call Py_IncRef(Py_None)