• Petr Viktorin's Python Stable ABI improvement notes
• PEP 489 -- Multi-phase extension module initialization
  • bpo-1635741: Py_Finalize() doesn't clear all Python objects at exit (convert extensions to multi-phase init PEP 489)
• PEP 620 -- Hide implementation details from the C API (Victor Stinner)
  • Move the default C API towards the limited C API
  • Make structures opaque:
    • PyObject: https://bugs.python.org/issue39573
    • PyTypeObject: https://bugs.python.org/issue40170
    • PyFrameObject: https://bugs.python.org/issue40421
    • PyThreadState: https://bugs.python.org/issue39947
    • PyInterpreterState: DONE in Python 3.8!
    • PyGC_Head: DONE in Python 3.9!
  • bpo-40989: Remove _Py_NewReference() and _Py_ForgetReference() from the public C API
  • bpo-41078: Convert PyTuple_GET_ITEM() macro to a static inline function
  • bpo-40601: Hide static types from the limited C API
• PEP 573 -- Module State Access from C Extension Methods
  • Python 3.10 has a new _PyType_GetModuleByDef() function
  • Python 3.9 added:
    • New PyType_FromModuleAndSpec()
    • New PyType_GetModuleState()
    • New METH_METHOD calling convention flag
    • New PyCMethod function signature
    • New defining_class type in Argument Clinic
• PEP 630 -- Isolating Extension Modules (Petr Viktorin)
  • bpo-40077: Convert static types to heap types: use PyType_FromSpec()
• bpo-41111: Convert a few stdlib extensions to the limited C API
• HPy project

Here is a short explanation. For a longer explanation, read A New C API for CPython (September 2017) by Victor Stinner.

Given the following code:

typedef struct {
    PyVarObject ob_base;
    PyObject **ob_item;   // <-- pointer to the array of list items
    Py_ssize_t allocated;
} PyListObject;

#define PyList_GET_ITEM(op, i) ((PyListObject *)op)->ob_item[i]

And the following C code:

PyObject *item = PyList_GET_ITEM(list, 0);

On a 64-bit machine, the machine code of a release build becomes something like:

PyObject **items = (PyObject **)(((char*)op) + 24);
PyObject *item = items[0];

whereas a debug build uses an offset of 40 instead of 24, because PyVarObject contains additional fields for debugging purpose:

PyObject **items = (PyObject **)(((char*)op) + 40);
PyObject *item = items[0];

As a consequence, the compiled C extension is incompatible at the ABI level: a C extension has to be build twice, once in release mode and once in debug mode.

To reduce the maintenance burden, :ref:`Linux vendors <os-vendors>` only provide C extensions compiled in release mode, making the :ref:`debug mode <debug-build>` mostly unusable on Linux in practice.

• CPython documentation: Stable Application Binary Interface
• Who uses it?
• PEP 384 -- Defining a Stable ABI by Martin v. Löwis: implemented in CPython 3.2 (2011)

• https://abi-laboratory.pro/tracker/timeline/python/
• https://bugs.python.org/issue21142
• https://sourceware.org/libabigail/