Extending Python with C or C++

[Nick Craig-Wood]

Ralf Schoenian <ralf at schoenian-online.de> wrote:
>  Ryan wrote:
> > I've been using Python for many years now. It's a wonderful language
> > that I enjoy using everyday. I'm now interested in getting to know
> > more about the guts (C/C++) and extending it. But, extending python
> > still seems like a black art to me. Is there anymore docs or info on
> > extending it besides the standard sparse ones (http://www.python.org/
> > doc/2.5.2/ext/intro.html) that may give me more insight? Is there a
> > class available? How can I learn more about the guts of python? How
> > would one go about following an interest in contributing to the
> > development of python.
> 
>  It is not exactly what you are looking for but nevertheless I am 
>  thinking the  article "Automatic C Library Wrapping -- Ctypes from the 
>  Trenches" may be interesting for you. You can find it in the latest 
>  Python Papers issue or simply following the link: 
>  http://ojs.pythonpapers.org/index.php/tpp/article/view/71

Interesting - I didn't know about h2xml and xml2py before and I've
done lots of ctypes wrapping!  Something to help with the initial
drudge work of converting the structures would be very helpful.

( http://pypi.python.org/pypi/ctypeslib/ )

I gave it a quick go and it worked fine.  I had to edit the XML in one
place to make it acceptable (removing a u from a hex number).  The
output of xml2py was an excellent place to start for the conversion,
though I don't think I'd want to use an automated process like in the
paper above as its output needed tweaking.

...

Here are my thoughts on the conversion :-

It converted an interface which looked like this (the inotify interface)

    struct inotify_event {
        int      wd;       /* Watch descriptor */
        uint32_t mask;     /* Mask of events */
        uint32_t cookie;   /* Unique cookie associating related
                              events (for rename(2)) */
        uint32_t len;      /* Size of name field */
        char     name[];   /* Optional null-terminated name */
    };

Into this

    class inotify_event(Structure):
        pass
    inotify_event._fields_ = [
        ('wd', c_int),
        ('mask', uint32_t),
        ('cookie', uint32_t),
        ('len', uint32_t),
        ('name', c_char * 0),
    ]

Which is a very good start.  However it defined these which clearly
aren't portable

    int32_t = c_int
    uint32_t = c_uint

Whereas it should have been using the ctypes inbuilt types

    c_int32
    c_uint32

Also I don't think c_char * 0 does anything sensible in ctypes,
c_byte * 0 is what is required plus a bit of casting.  This is a
non-standard GNU extension to C though.

All that said though, it looks like a great time saver.

-- 
Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick