[Python-Dev] Tagged integers

[Phillip J. Eby]

At 08:40 AM 7/14/04 -0700, Guido van Rossum wrote:
> > I'd rather not like to see hackery like misused pointers
> > in the core, so you can count me as -1 on this.
>
>-1000 here.  In a past life (the ABC implementation) we used this and
>it was a horrible experience.  Basically, there were too many places
>where you had to make sure you didn't have an integer before
>dereferencing a pointer, and finding the omissions one core dump at a
>time was a nightmare.

That certainly seems like it would be so, except...  doesn't every bit of 
Python code that's doing anything *other* than accessing ob_type or 
ob_refcnt have to first check if ob_type is the type that code is looking 
for anyway?

And, don't the vast majority of accesses to ob_type and ob_refcnt occur 
inside Python core API macros and functions anyway?

If in addition to Mr. Knight's patch, ob_type and ob_refcnt were renamed so 
as to break any existing direct access in the core or extension modules, 
upgrading them to use Py_GETTYPE and Py_GETREF would be straightforward 
because one would "find the omissions" one *compilation error* at a time.

Further, renaming those fields to something like 'XXXusePy_GETTYPE_instead' 
would provide a built-in hint that you weren't supposed to use the field 
directly.  :)

An additional trick: make it so that the macros for defining new object 
types still create an ob_type field, but let PyObject * point to the 
structure with the XXX fields.  Thus, you can only access ob_type directly 
if you've already cast to a non PyObject * type.  (i.e., you've already 
used an appropriate PyXXX_Check on that variable and would like to use it 
as a regular object now).

Naturally, the issues of portability and what impact Mr. Knight's approach 
has on non-integer code would need to be taken into consideration as well, 
but it seems to me that this approach *could* be made type-safe, at the 
cost of some initial pain to update the approximately 640 uses of 'ob_type' 
in the current source base.