[Python-Dev] Let's update CObject API so it is safe and regular!

[Brett Cannon]

On Tue, Mar 31, 2009 at 20:34, Guido van Rossum <guido at python.org> wrote:

> Can you get Jim Fulton's feedback? ISTR he originated this.
>

I thought Neal started this idea?

-Brett


>
> On Tue, Mar 31, 2009 at 12:14 PM, Larry Hastings <larry at hastings.org>
> wrote:
> >
> > The CObject API has two flaws.
> >
> > First, there is no usable type safety mechanism.  You can store a void
> > *object, and a void *description.  There is no established schema for
> > the description; it could be an integer cast to a pointer, or it could
> > point to memory of any configuration, or it could be NULL.  Thus users
> > of the CObject API generally ignore it--thus working without any type
> > safety whatsoever.  A programmer could crash the interpreter from pure
> > Python by mixing and matching CObjects from different modules (e.g. give
> > "curses" a CObject from "_ctypes").
> >
> > Second, the destructor callback is defined as taking *either* one *or*
> > two parameters, depending on whether the "descr" pointer is non-NULL. One
> > can debate the finer points of what is and isn't defined behavior in
> > C, but at its heart this is a sloppy API.
> >
> > MvL and I discussed this last night and decided to float a revision of
> > the API.  I wrote the patch, though, so don't blame Martin if you don't
> > like my specific approach.
> >
> > The color of this particular bike shed is:
> > * The PyCObject is now a private data structure; you must use accessors.
> >  I added accessors for all the members.
> > * The constructors and the main accessor (PyCObject_AsVoidPtr) now all
> >  *require* a "const char *type" parameter, which must be a non-NULL C
> >  string of non-zero length.  If you call that accessor and the "type"
> >  is invalid *or doesn't match*, it fails.
> > * The destructor now takes the PyObject *, not the PyCObject *.  You
> >  must use accessors to get your hands on the data inside to free it.
> >
> > Yes, you can easily skip around the "matching type" restriction by
> > calling PyCObject_AsVoidPtr(cobj, PyCObject_GetType(cobj)).  The point
> > of my API changes is to *encourage* correct use.
> >
> > I've posted a patch implementing this change in the 3.1 trunk to the
> > bug tracker:
> >
> >   http://bugs.python.org/issue5630
> >
> > I look forward to your comments!
> >
> >
> > /larry/
> >
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>
>
>
> --
> --Guido van Rossum (home page: http://www.python.org/~guido/<http://www.python.org/%7Eguido/>
> )
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