[capi-sig] Python 3.x PyObject_HEAD versus PyObject_VAR_HEAD

[M.-A. Lemburg]

Philip Semanchuk wrote:
> 
> On Feb 1, 2010, at 5:27 AM, M.-A. Lemburg wrote:
>> Perhaps you are confusing this with PyVarObject_HEAD_INIT() ?!
> 
> 
> THanks for your reply. You're right, I should have said that it is
> PyVarObject_HEAD_INIT that's used hundreds of times while
> PyObject_HEAD_INIT is hardly used at all (according to `grep -r`).
> Actually, I can state that even more strongly: PyObject_HEAD_INIT is
> only used once -- in the #define for PyVarObject_HEAD_INIT.
> 
> After spending another hour reading the Python source, I think I get it
> now. As is often the case, the problem came from not reading carefully.
> When I saw this:
> 
>    PyTypeObject PyFloat_Type = {
>        PyVarObject_HEAD_INIT(&PyType_Type, 0)
>        "float",
>        sizeof(PyFloatObject),

Right, it's being used to init the (static) type object PyFloat_Type
in the above case.

> I saw PyVarObject_HEAD_INIT being used to initialize a PyFloatObject,
> which is not the case -- it's initializing a PyTypeObject. Since the
> PyTypeObject struct definition uses PyObject_VAR_HEAD, it makes sense
> that PyVarObject_HEAD_INIT is used to initialize it.
> 
> That also explains why PyVarObject_HEAD_INIT shows up so much in the
> Python source -- it's all those PyTypeObjects getting initialized.

Right.

> This brings up two more questions that I could probably answer with more
> reading, but if you have time for some further explanation I'd
> appreciate it. First, in what context would one use PyObject_HEAD_INIT?
> Second, why does the PyTypeObject struct definition use
> PyObject_VAR_HEAD when a type object has no concept of length?

It does: additional members can be stored in the variable part of the
object, ie. low-level attributes which are exposed as slots in
Python.

-- 
Marc-Andre Lemburg
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