[Python-3000] PyObject_HEAD_INIT

[M.-A. Lemburg]

On 2008-11-20 20:34, Roger Binns wrote:
> M.-A. Lemburg wrote:
>> Whether you write:
> 
>> {PyObject_HEAD_INIT(0), 0, ...
> 
>> or
> 
>> {PyVarObject_HEAD_INIT(0, 0), ...
> 
>> for your type definition doesn't really make much difference. 
> 
> Actually in Py 3 it does.  If you use the former (which is how Py 2 does
> it) then you get serious compiler warnings due to misaligned fields in
> Py 3 and presumably even worse if run the code. 

You might get warnings (esp. from GCC), but I have yet to see a compiler
that doesn't map the above to the same memory.

After all, Python 2 has been using this layout for years without any
compiler warnings or segfaults because of this.

> See PEP 3123 as to why
> things changed.  That is why all the code in Python 3 was changed from
> using the former to the latter.

Right. Things are now more standard compliant and you get fewer
warnings.

>> However, the macros exist to be used for both variable size
>> and fixed size objects, so having both available is useful and
>> legitimate.
> ...
>> Except maybe a few thousand extensions already using it which are
>> waiting to be ported to Python 3.
> 
> Can you point to any?  All the ones I found (via Google) only use
> PyObject_HEAD_INIT for PyTypeObjects and every single one of those will
> have to change to using PyVarObject_HEAD_INIT.

True, because PyTypeObjects *are* in fact PyVarObjects and not
PyObjects, so they should have used PyVarObject_HEAD_INIT all
along.

It's only that compilers never really cared or always
did the right thing - depending on how you see it :-)

BTW: With the "few thousand extensions" I was referring to the current
use of the PyObject_HEAD_INIT() macro which you wanted to remove, not
to a few thousand extensions using it correctly. Note that it's rather
uncommon to create singletons like the type objects in C.

-- 
Marc-Andre Lemburg
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