Hi all,
I'm trying to port my posix_ipc extension to Python 3, and I'm
confused by the documentation for PyObject_HEAD and PyObject_VAR_HEAD.
My understanding is that a PyObject-derived type should begin its
PyTypeObject struct with PyObject_HEAD and a PyVarObject-derived type
with PyObject_VAR_HEAD. the documentation says of PyVarObject, "This
type does not often appear in the Python/C API."
http://docs.python.org/3.1/c-api/structures.html#PyVarObject
The doc also says that PyObject_VAR_HEAD, "is used when declaring new
types which represent objects with a length that varies from instance
to instance."
http://docs.python.org/3.1/c-api/structures.html#PyObject_VAR_HEAD
I assumed, then, that PyObject_VAR_HEAD would also appear
infrequently, but that's not the case at all. In fact, I can't find
PyObject_HEAD used at all in the Python 3.1.1 source code, whereas
PyObject_VAR_HEAD is used hundreds of times, including for objects
that don't have a length (like the float type, for instance).
Is the documentation wrong, or am I misreading it? Suggestions
appreciated.
THanks Philip
Philip Semanchuk wrote:
Hi all, I'm trying to port my posix_ipc extension to Python 3, and I'm confused by the documentation for PyObject_HEAD and PyObject_VAR_HEAD.
My understanding is that a PyObject-derived type should begin its PyTypeObject struct with PyObject_HEAD and a PyVarObject-derived type with PyObject_VAR_HEAD. the documentation says of PyVarObject, "This type does not often appear in the Python/C API." http://docs.python.org/3.1/c-api/structures.html#PyVarObject
The doc also says that PyObject_VAR_HEAD, "is used when declaring new types which represent objects with a length that varies from instance to instance." http://docs.python.org/3.1/c-api/structures.html#PyObject_VAR_HEAD
I assumed, then, that PyObject_VAR_HEAD would also appear infrequently, but that's not the case at all. In fact, I can't find PyObject_HEAD used at all in the Python 3.1.1 source code, whereas PyObject_VAR_HEAD is used hundreds of times, including for objects that don't have a length (like the float type, for instance).
I'm not sure how you searched for these, but I get somewhat different counts:
33 PyObject_HEAD 10 PyObject_VAR_HEAD
in the Include/ dir of Python 3 (SVN)
Is the documentation wrong, or am I misreading it? Suggestions appreciated.
The docs are correct.
PyObject_HEAD does not include the field ob_size, PyObject_VAR_HEAD does.
Perhaps you are confusing this with PyVarObject_HEAD_INIT() ?!
On Feb 1, 2010, at 5:27 AM, M.-A. Lemburg wrote:
Philip Semanchuk wrote:
Hi all, I'm trying to port my posix_ipc extension to Python 3, and I'm
confused by the documentation for PyObject_HEAD and PyObject_VAR_HEAD.My understanding is that a PyObject-derived type should begin its PyTypeObject struct with PyObject_HEAD and a PyVarObject-derived type with PyObject_VAR_HEAD. the documentation says of PyVarObject, "This type does not often appear in the Python/C API." http://docs.python.org/3.1/c-api/structures.html#PyVarObject
The doc also says that PyObject_VAR_HEAD, "is used when declaring new types which represent objects with a length that varies from
instance to instance." http://docs.python.org/3.1/c-api/structures.html#PyObject_VAR_HEADI assumed, then, that PyObject_VAR_HEAD would also appear
infrequently, but that's not the case at all. In fact, I can't find PyObject_HEAD
used at all in the Python 3.1.1 source code, whereas PyObject_VAR_HEAD is used hundreds of times, including for objects that don't have a
length (like the float type, for instance).I'm not sure how you searched for these, but I get somewhat different counts:
33 PyObject_HEAD 10 PyObject_VAR_HEAD
in the Include/ dir of Python 3 (SVN)
Is the documentation wrong, or am I misreading it? Suggestions
appreciated.The docs are correct.
PyObject_HEAD does not include the field ob_size, PyObject_VAR_HEAD does.
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),
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.
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?
Cheers Philip
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.
-
M.-A. Lemburg -
Philip Semanchuk