[C++-sig] boost::python embedding error handling

[Jeffrey Holle]

The good news is that now I can get useful error messages from 
boost::python, I'm more than willing to use it.  Its a keeper!

I would like to provide some critism of the 
boost::python::error_already_set exceptions.

1.  It doesn't inherit from std::exception.

2.  It provides no information.

On the other hand, its concept of obtaining error information from the 
python interpreter is soundly a good idea.  Its just isn't finished.

I've written what I would consider a replacement for it and attached it.
Note that the code it its constuctor is pretty much what Chad Austin 
contributed to this news group.  Thanks.

To use it, I will need to place a try clause around all boost::python 
embedding code with the following catch clause:

  catch (error_already_set&) {
    throw python_error();
  }

I consider this acceptable, but it would be better to not need it.

In critism of python_error:

1.  It inherits from std::runtime_error.  It can be argued, I think, 
that it should be std::exception instead.  Becaue std::runtime_error 
doesn't provide protected access to its string attribute, I've had to 
provide one in python_error.  This means there is need for a "what" 
method implementation and that runtime_error's attribute is wasted bagage.

2.  It arguably does too much in its constructor.  This is a convenence 
issue for me.  Constructed at a lower level, it could easily be changed.

3.  Chad's code assumes that the python objects returned from 
PyErr_Fetch are strings.  I do not know if this assumption is safe. 
I've seen code on the web that does not make it.

4.  As Chad indicated, his code presently doesn't handle the traceback 
info.  I plan to add this to python_error as a seperate attribute and 
accessor, eventually ...


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