How to catch a usefull error message ?

[Vincent Vande Vyvre]

Le 23/04/19 à 21:48, MRAB a écrit :
> On 2019-04-23 19:21, Vincent Vande Vyvre wrote:
>> Le 23/04/19 à 19:23, Chris Angelico a écrit :
>>> On Wed, Apr 24, 2019 at 3:18 AM Vincent Vande Vyvre
>>> <vincent.vande.vyvre at telenet.be> wrote:
>>>> Hi,
>>>>
>>>> In a CPython lib I have an _init() method wich take one argument, a 
>>>> file
>>>> name.
>>>>
>>>>       char *fname;
>>>>
>>>>       if (!PyArg_ParseTuple(args, "s", &fname))
>>>>           return NULL;
>>>>
>>>> So, if I instanciate my object with a bad argument I've a good error
>>>> message:
>>>>
>>>> tif = ImgProc(123)
>>>> TypeError: argument 1 must be str, not int
>>>> (followed by the traceback)
>>>>
>>>> But if I do:
>>>> try:
>>>>       tif = ImgProc(123)
>>>> except Exception as why:
>>>>       print("Error:", why)
>>>>
>>>> I get just:
>>>>
>>>> Error: <class '_liboqapy.ImgProc'> returned a result with an error set
>>>>
>>> It looks like there's an internal problem in the C function. Are you
>>> sure it's hitting the PyArg_ParseTuple and then immediately returning
>>> NULL? Post a bit more of your code; this error looks like something is
>>> leaving an error state but then carrying on with the code.
>>>
>>> ChrisA
>>
>> Into the lib:
>>
>> static int
>> ImgProc_init(ImgProc *self, PyObject *args, PyObject *kwds)
>> {
>>       PyObject *tmp;
>>       char *fname;
>>
>>       if (!PyArg_ParseTuple(args, "s", &fname))
>>           return NULL;
>>
>>       tmp = self->src;
>>       self->src = PyUnicode_FromString(fname);
>>       Py_XDECREF(tmp);
>>       return 0;
>> }
>>
> [snip]
>
> That function returns an int.
>
> If PyArg_ParseTuple fails, your function returns NULL, which is cast 
> to an int, 0.
>
> If PyArg_ParseTuple succeeds, your function returns 0.
>
> Either way, it returns 0.
>
> So how does the caller know whether the function was successful? Does 
> it check PyErr_Occurred?

No, the caller is in a block try-except for that.

The exact question here is why without a try-except I've the good one 
error and not in a try-except.

The /return 0;/ is usual in a /Foo_init()/ function.


Vincent