PyTypeObject.ob_type is not available in cpython

[samuelcolvin]

Bug Description

I'm trying to access PyTypeObject.ob_type, but it doesn't seem to be available on cpython:

pyo3/pyo3-ffi/src/cpython/object.rs

Lines 221 to 222 in 18081ba

	#[cfg(all(PyPy, not(Py_3_9)))]
	pub ob_type: *mut PyTypeObject,

I believe it should be available, especially since value.get_type().get_type_ptr() works fine.

According to the cpython api docs I think it should be available.

Steps to Reproduce

I would expect the following to pass, but it fails to compile

let value: &PyAny = ...
let type_ptr = value.get_type_ptr();
let meta_type = unsafe { (*type_ptr).ob_type };
assert_eq!(meta_type, value.get_type().get_type_ptr());

! Note, I've tried this with pypy 3.8 and it passes.

Backtrace

No response

Your operating system and version

Ubuntu

Your Python version (python --version)

Python 3.10.4

Your Rust version (rustc --version)

cargo 1.68.0-nightly (2381cbdb4 2022-12-23)

Your PyO3 version

v0.18.0

How did you install python? Did you use a virtualenv?

pyenv

Additional Info

No response

[davidhewitt]

So the CPython doc you link is actually for PyObject, not PyTypeObject. Since PEP 3123 PyTypeObject contains PyObject as the first field (well, actually PyVarObject, which itself contains PyObject as the first field), So you could write this as PyTypeObject.ob_base.ob_base.ob_type. Or the easier option, and the point of PEP 3123, is that you can just cast to PyObject * and then read ob_type directly.

PyPy before 3.9 was not PEP 3123 compliant (and still isn't on 3.9 windows, I'm not sure what's going on there), so the source line you link to is just to support old pypy versions.

So in summary, this is working as intended

[davidhewitt]

I think the following can work to get the metaclass safely (on my phone, so haven't tested):

let value: &PyAny = ...
let type_ = value.get_type();
let meta_type = type_.get_type();

[samuelcolvin]

Thanks, (*type_ptr).ob_base.ob_base.ob_type is what I needed, see pydantic/pydantic-core#373

[mejrs]

For illustration, an instance of a custom Python class' layout looks something like this:

PyObject @ 0x14e93a9ed40 {
	ob_refcnt: 1,
	ob_type: PyTypeObject @ 0x14e9334b2b0 {
		ob_base: PyVarObject @ 0x14e9334b2b0 {
			ob_base: PyObject @ 0x14e9334b2b0 {
				ob_refcnt: 5,
				// 'object'
				ob_type: PyType_Type @ 0x7ffe3789b920 {
                                      // all the fluff inside `object`
                                }
			},
			ob_size: 0,
		},
		tp_name: "MyClass" @ 0x14e93a73660,
		tp_basicsize: 32,
		tp_itemsize: 0,
// etc