Print traceback in C code

[Thomas Heller]

Mark Hammond <mhammond at skippinet.com.au> writes:

> Thomas Heller wrote:
> > I'm trying to print a traceback in C code.
> > According to some articles I found, mainly by Mark Hammond, it's
> 
> > basically that I would have to write something like this in C
> >   import traceback, StringIO
> 
> >   s = StringIO.StringIO()
> >   traceback.print_exception(tb_type, tb_value, tb_traceback, file=s)
> > and then call s.get_value() and convert it to a string.
> 
> > I'm stuck, however, because calling
> >   PyErr_Fetch(&tb_type, &tb_value, &tb_traceback);
> 
> > leaves a NULL pointer in tb_traceback, although tb_type and tb_value
> 
> > are Python objects. Even calling PyErr_NormalizeException() doesn't
> > help.
> >
> 
> 
> I believe that the traceback is setup by the exception handling code
> in Python popping out the stack frames.  A traceback is from the
> "current frame" to the frame causing the error, and obviously the
> depth of this changes as exception handlers at different levels are
> walked.
> 
> 
> So, if your C code is setting an exception, then trying to print it,
> there is no traceback available.  However, if your C code called into
> Python, and Python returned NULL indicating an exception, then Python
> will have set the traceback up as a result of the normal processing
> that returned control to your code from the API.

Yes, this makes sense. In other words, if my C code sets the exception,
there *are* no stack frames.

In this case (which now mysteriously doesn't happen any longer),
I pass Py_None to the print_exception call and everything is fine.

Thanks,

Thomas