How to catch a usefull error message ?

[Vincent Vande Vyvre]

Le 24/04/19 à 19:57, MRAB a écrit :
> 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.

In this case yes, beause it need to return the result of the command system.

But the "return 0" is a common case for an "Foo_init()"

see: 
https://docs.python.org/3.5//extending/newtypes.html#adding-data-and-methods-to-the-basic-example

... And that's nothing to do with my initial question

Vincent