Hi Giuseppe,
On 11/06/2012 10:08 AM, Giuseppe Corbelli wrote:
On 05/11/2012 18:51, Martin Hellmich wrote:
Hi,
I have a custom exception in C++ that I would like to use in Python.
Let's call it MyException with custom constructor and members (an
implementation is below).
I would like to expose it to python so that I can raise it:
> raise MyException(-1, 'tragic error')
The perfect way that I can imagine is to tell boost::python to base
the class
on PyExc_Exception, but that doesn't seem to work. Furthermore I have
found
various solutions how to translate exceptions thrown in C++ into
proper Python
exceptions, but in my case these solutions sit at the wrong place.
I've based my code on.
http://stackoverflow.com/questions/9620268/boost-python-custom-exception-class
I have used the same reference and also this suggestion for including
the other attributes shown here:
http://stackoverflow.com/questions/11448735/boostpython-export-custom-exception-and-inherit-from-pythons-exception
However, this doesn't solve my problem.
It works fine when I throw the MyException in C++, but when I raise the
MyCPPException in Python, the translator is not called (which makes
sense), so the attribute 'cause' does not change.
Actually, before I throw the MyException for the first time in C++,
MyCPPException does not have the attribute 'cause' at all.
This issue should be the same, if I set the exception with
PyErr_SetObject, since the translator is not called, when I 'raise' it.
My current attempt is quite different (and all in Python):
import exc
class MyException(Exception):
def __init__(self, *args):
self.e = exc.MyException(*args)
def __getattr__(self, name):
return self.e.__getattribute__(name)
def __str__(self):
return type(self).__name__ + ': ' + self.e.what()
e=MyException(1, 'bla')
I can 'raise MyException(1, 'some error')' and get the attributes I have
in C++. If the attributes change, it should all work except for the
__str__ function, which is hardcoded.
The drawback is that it's in python. The python part will be developed
by third parties and I then have to make sure that they see and use this
provided exception ...
Also I don't know yet how translating this back to C++ will play out.
I guess I am jumping a bit ahead ... is it correct from me to assume
that this behaviour is not possible with the solutions posted on
stackoverflow or am I overlooking something?
I am very happy about any replies :)
Cheers
Martin
I would like to avoid to create a corresponding python class to the C++
exception, because there would be added effort to keep the two
descriptions
consistent.
True.
class MyException: public std::exception {
MyException();
MyException(int code, const std::string &string);
int code();
const char* what();
int errorCode;
std::string errorMessage;
};
The only unusual stuff is that you need both code and message. 2 ideas
come to mind.
1) you can base the python exception not on PyExc_Exception but on a
custom class which can handle both error code and message, so the
exception translator can do both a PyErr_SetString and a
"PyErr_SetCode". This way you can use the exception in both C and Python
in the same style, no need to keep anything in sync.
2) you can base the Python exception on the standard PyExc_Exception but
use PyErr_SetObject in the exception translator. A quick search finds this
http://stackoverflow.com/questions/2261858/boostpython-export-custom-exception
(does not inherit from Exception, though)
--
Martin Hellmich Information Technology Department
mhell...@cern.ch CERN
+41 22 76 765 26 CH-1211 Geneva 23
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