I propose to investigate an incompatible C API change: make thePyTypeObject andPyHeapTypeObject structures opaque, remove their members from the public C API (move them to the internal C API). We have to investigate how it's outside in 3rd party C extensions (ex: Cython, pybind11, etc.), and design a smooth migration plan.

The PyTypeObject structure is exposed as part of the public Python C API. For example,Py_TYPE(obj)->tp_name directly gets a type name (as a UTF-8 encoded byte string,char*).

The PyTypeObject members areNOT part of the limited C API (PEP 384).

In Python 3.9 (2020), I reworked the C API to avoid accessing directly PyTypeObject members at the ABI level: issue#84351. For example, Python 3.8 implementsPyObject_IS_GC() as a macro:

/* Test if an object has a GC head */#definePyObject_IS_GC(o) \    (PyType_IS_GC(Py_TYPE(o)) \     && (Py_TYPE(o)->tp_is_gc == NULL || Py_TYPE(o)->tp_is_gc(o)))

whereas Python 3.9 only provides an opaque function call:

/* Test if an object implements the garbage collector protocol */PyAPI_FUNC(int)PyObject_IS_GC(PyObject*obj);

At the ABI level, the direct access to thePyTypeObject.tp_is_gc member became an opaque function call.

Changing PyTypeObject API and ABI caused a lot of troubles in the past. Example:

• issueC files generated by Cython set tp_print to 0: PyTypeObject.tp_print removed #81431: Cython broken byPyTypeObject.tp_print removal -- related toPyTypeObject.tp_vectorcall_offset addition
• issueRemove cross-version binary compatibility #76569: ABI issue with the addition ofPyTypeObject.tp_finalize (PEP 442). IsPy_TPFLAGS_HAVE_FINALIZE needed for ABI compatibility? See also thepython-dev thread.
• issueRemove Py_TPFLAGS_HAVE_VERSION_TAG flag? #86913: removePy_TPFLAGS_HAVE_VERSION_TAG flag (ABI compatibility)
• issueDocumentation inconsistency with the stable ABI #91271: Documentation inconsistency with the stable ABI (PyTypeObject)
• ThePEP 620 uses PyTypeObject incompatible changes in its rationale.

For many years, there is a work-in-progress to convert all Python built-in types and types of stdlib extensions from static types to heap types. See for example issue#84258 andPEP 630.

The API and ABI for heap type was also enhanced over the years. Examples:

• Python 3.12 addsPyType_FromMetaclass()
• Add the buffer protocol to the limited C API (to PyType_FromSpec()): issueLimited API support for Py_buffer #89622
• issue[C API] PyType_GetSlot cannot get tp_name #86201: get tp_name with PyType_GetSlot()

In the past, other structure members were removed:

• PyInterpreterState: Python 3.8
• PyGC_Head: Python 3.9
• PyFrameObject: Python 3.11

The work was also prepared for:

• PyObject: issue[C API] Avoid accessing PyObject and PyVarObject members directly: add Py_SET_TYPE() and Py_IS_TYPE(), disallow Py_TYPE(obj)=type #83754, Python 3.10 and Python 3.11, add Py_SET_REFCNT() and Py_SET_TYPE()
• PyTheadState: issue[C API] Prepare the C API to make PyThreadState opaque: add getter functions #84128, Python 3.11