[C++-sig] Re: Presenting a Python embedding tutorial for Boost.Python

David Abrahams dave at boost-consulting.com
Mon Dec 9 03:38:04 CET 2002

Dirk Gerrits <dirk at gerrits.homeip.net> writes:

> David Abrahams wrote:
>> Dirk Gerrits <dirk at gerrits.homeip.net> writes:
>>>I've finally finished the first draft of the embedding tutorial I was
>>>writing for Boost.Python. I guess I underestimated the task, because
>>>it took several days longer than I anticipated, but here it is:
>>>Note that I am neither an experienced writer, nor an experienced
>>>Pythoneer. Any and all proof reading will be greatly appreciated.
>> Thanks very much for your efforts!
> Your welcome. I'm just glad I can help a great library become even
> better. :)

Many <g>s!

>> I have several high-level remarks:
>> 1. Why spend so much time discussing manual reference-counting, if we
>>    have a better solution?
> Good point. At the time, I felt it was necessary to understand this
> to understand the difference between 'borrowed' and 'new'. On
> retrospect, I suppose this is not really necessary.

I don't know, maybe it is neccessary. It depends on whether you think
your audience understands how to use the Python 'C' API or not.  If
you think many don't, it might be best to write a separate tutorial
section on how to use handle<>. I know, the project is growing ;-)
OTOH you've already written much of it.
>> 2. I notice no use of handle_exception(). Boost.Python code can throw
>>    C++ exceptions which correspond to Python exceptions (e.g. when
>>    initializing a handle<> from a null pointer), and
>>    handle_exception() will translate these back into Python
>>    exceptions... which can finally be printed on the console or
>>    wherever else you think is appropriate
> Well I guess that is because I haven't studied Boost.Python's
> exception facilities in much detail myself. But I agree that this
> would make a good addition. And a description of null_ok would
> probably be appropriate too.

That part should go in the handle<> tutorial ;-)
>> 3. It was always my intention to help people avoid touching PyObject*s
>>    and the Python 'C' API at all.  Are there any obvious C++
>>    interfaces that should be added to the library to wrap the
>>    boilerplate use of the Python 'C' API I see in your examples?
> A noble goal. :) And you've already got dict, list, long, numeric,
> object, str, tuple which covers a great deal of ground. My PyTuple_
> examples are perhaps ill-chosen because python::tuple can do this much
> easier

Yes, it really would be best to avoid touching PyTuple_xxx directly.

> , but I didn't know what else to demonstrate there.

I also confess to not having any idea what piece of code you're
talking about ;-)

See also http://www.mcmillan-inc.com/embed.html, FWIW.

> If I scrap the manual reference counting section though, then I've
> still got the PyRun_String example. The PyRun_ functions are pretty
> vital for embedding and they are not directly covered by Boost.Python
> However, adding Boost.Python facilities for the PyRun_ functions
> probably means adding a load of other Python C API facilites as well.

Yup. It was always the intention to add these. Dave Hawkes made a
valiant effort:
http://mail.python.org/pipermail/c++-sig/2002-June/001503.html, but it
was in some ways just too much to take into the library at once.

> I would like to have something like this in Boost.Python:
> object run_python_code(InputIterator begin, InputIterator end);

These must have char as their value_type?

> object run_python_code(String str);

Umm, what type is String?

> Or, if a python::module was added, perhaps this would be more generic:
> object run_python_code(InputIterator begin, InputIterator end, module
> mod = main_module());
> object run_python_code(String str, module mod = main_module());

We don't really need a new type for that; we /could/ just use
object. Until module has some useful constructor or member functions,
there's no reason to add it.

> Also, it might be a good idea to wrap Py_Initialize and Py_Finalize in
> a RAII class?

Absolutely. Just as soon as Py_Finalize becomes safe for us ;-)
That might be a good project for you...

> I'm just not sure where this line of thought will end. It might add a
> lot of functions and classes to Boost.Python.
> What do you think?

I think that's OK, as long as we take small steps.

We could start with just the minimum needed to do the kinds of
embedding jobs you're tackling here.

                       David Abrahams
   dave at boost-consulting.com * http://www.boost-consulting.com
Boost support, enhancements, training, and commercial distribution

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