[Python-Dev] [capi-sig] Exceptions with additional instance variables

Mon Dec 22 13:48:46 CET 2008

On Mon, Dec 22, 2008 at 10:45 AM, Guilherme Polo <ggpolo at gmail.com> wrote:
> On Mon, Dec 22, 2008 at 10:06 AM, <chojrak11 at gmail.com> wrote:
>> On Mon, Dec 22, 2008 at 03:29, Guilherme Polo <ggpolo at gmail.com> wrote:
>>> On Sun, Dec 21, 2008 at 11:02 PM, <chojrak11 at gmail.com> wrote:
>>>> Hello,
>>>>>>>> I'm trying to implement custom exception that have to carry some
>>>> useful info by means of instance members, to be used like:
>>>>>>>> try:
>>>> // some code
>>>> except MyException, data:
>>>> // use data.errorcode, data.errorcategory, data.errorlevel,
>>>> data.errormessage and some others
>>>>>>>> The question is - how to implement the instance variables with
>>>> PyErr_NewException?
>>>>>> Using PyErr_NewException is fine. You must understand that an
>>> exception is a class, and thus PyErr_NewException creates one for you
>>> and returns it.
>>> Just like you would do with a class that has __dict__, set some
>>> attributes to what you want. That is, use PyObject_SetAttrString or
>>> something more appropriated for you.
>>>> Ok so I did the following. In init function (forget refcounting and
>> error checking for a moment ;-)
>>>> PyObject *dict = PyDict_New();
>> PyDict_SetItemString(dict, "errorcode", PyInt_FromLong(0));
>> static PyObject *myexception =
>> PyErr_NewException("module.MyException", NULL, dict);
>> You do not really have to create a dict here, one will be created for
> you if you pass a NULL there.
>>> PyModule_AddObject(module, "MyException", myexception);
>>>> It worked more or less as expected, the help shown:
>>>> | ----------------------------------------------------------------------
>> | Data and other attributes defined here:
>> |
>> | errorcode = 0
>> |
>> | ----------------------------------------------------------------------
>>>> Then I did the following when raising the exception:
>>>> PyObject_SetAttrString(myexception, "errorcode", PyInt_FromLong(111));
>> PyErr_SetString(myexception, "Bad thing happened");
>> return NULL;
>>>> and the test code was:
>> try:
>> do_bad_thing();
>> except MyException, data:
>>>> and you surely already guessed it -- data.errorcode was 0.... Not only
>> that, module.MyException.errorcode was also 0...
>>>> What I'm doing wrong? I certainly don't get the idea of exceptions in
>> Python, especially what is being raised - a class or an instance?
>> There are two forms raise can take, both will end up involving a class
> and a intsance.
>>> If
>> the latter - how's the class instantiated?
>> You can call a class to instantiate it.
>>> If not - what about values
>> in different threads? The docs are so vague about that...
>>>>>> Thanks again in advance,
>> Chojrak
>>>> Again, an exception is a class, so you could create a new type in C,
> and do anything you wanted. But you probably don't want to create a
> new type to achieve this

By creating a type I mean one that involves defining a tp_init, and
everything else your type needs, not about the simple one created by
PyErr_NewException.
> , so there are two simple ways I'm going to
> paste below:
>> #include "Python.h"
>> static PyObject *MyErr;
>> static PyMethodDef module_methods[] = {
> {"raise_test", (PyCFunction)raise_test, METH_NOARGS, NULL},
> {NULL},
> };
>> PyMODINIT_FUNC
> initfancy_exc(void)
> {
> PyObject *m;
>> m = Py_InitModule("fancy_exc", module_methods);
> if (m == NULL)
> return;
>> MyErr = PyErr_NewException("fancy_exc.err", NULL, NULL);
>> Py_INCREF(MyErr);
> if (PyModule_AddObject(m, "err", MyErr) < 0)
> return;
> }
>> the raise_test function is missing, pick one of these:
>> static PyObject *
> raise_test(PyObject *self)
> {
> PyObject_SetAttrString(MyErr, "code", PyInt_FromLong(42));
> PyObject_SetAttrString(MyErr, "category", PyString_FromString("nice one"));
> PyErr_SetString(MyErr, "All is good, I hope");
> return NULL;
> }
>> or
>> static PyObject *
> raise_test(PyObject *self)
> {
>> PyObject *t = PyTuple_New(3);
> PyTuple_SetItem(t, 0, PyString_FromString("error message"));
> PyTuple_SetItem(t, 1, PyInt_FromLong(10));
> PyTuple_SetItem(t, 2, PyString_FromString("category name here"));
> PyErr_SetObject(MyErr, t);
> Py_DECREF(t);
> return NULL;
> }
>> In this second form you check for the args attribute of the exception.
>> --
> -- Guilherme H. Polo Goncalves
>
-- 
-- Guilherme H. Polo Goncalves