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

Larry Hastings larry at hastings.org
Tue Mar 31 21:14:57 CEST 2009

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:


I look forward to your comments!


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