How to catch a usefull error message ?

[MRAB]

On 2019-04-23 20:21, Vincent Vande Vyvre wrote:
> Le 23/04/19 à 20:54, Chris Angelico a écrit :
>> On Wed, Apr 24, 2019 at 4:47 AM Vincent Vande Vyvre
>> <vincent.vande.vyvre at telenet.be> wrote:
>>
>> 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;
>> }
>>
>> If i do:
>>       try:
>>           tif = ImgProc(123)
>>       except Exception as why:
>>           print(sys.exc_info())
>>           raise
>> I get:
>> (<class 'SystemError'>, SystemError("<class '_liboqapy.ImgProc'>
>> returned a result with an error set",), <traceback object at
>> 0x7f3bcac748c8>)
>> TypeError: argument 1 must be str, not int
>>
>> The above exception was the direct cause of the following exception:
>>
>> Traceback (most recent call last):
>>     File "/home/vincent/oqapy-3/trunk/filters/ui_lenscorrection.py", line
>> 104, in on_main_cursor_changed
>>       self.prepare_preview_process()
>>     File "/home/vincent/oqapy-3/trunk/filters/ui_lenscorrection.py", line
>> 137, in prepare_preview_process
>>       self.main.process_on_preview(params)
>>     File "/home/vincent/oqapy-3/trunk/filters/lenscorrection.py", line
>> 56, in process_on_preview
>>       tif = ImgProc(123)
>> SystemError: <class '_liboqapy.ImgProc'> returned a result with an error set
>> --------------------------------------------------------------------------------
>> Why a SystemError ?
>> The SystemError means that you're using the Python C API in a way that
>> doesn't make sense to the interpreter. You're leaving a marker saying
>> "hey, I need you to throw an exception" but then you're also returning
>> a value. You'll need to figure out where that's happening and exactly
>> what is being called. How are you setting up your class?
>>
>> ChrisA
> 
> The syntaxe
> 
>       if (!PyArg_ParseTuple(args, "s", &fname))
>            return NULL;
> 
> Is the usage described in the doc [*]
> 
> And without block try-except I get the good one error.
> 
> 
> [*]
> https://docs.python.org/3.5//extending/extending.html#back-to-the-example
> 
If you look at the previous example, the function's return type is 
"PyObject *".

On success it returns a reference (pointer) to an object; on error it 
returns NULL.

Your function's return type is int.