Hi,

In 2017, I added a new experimental FASTCALL calling convention. See my articles about it: The start of the FASTCALL project and FASTCALL microbenchmarks. It avoids the need to create a temporary tuple to pass positional arguments and the need to create a temporary dictionary to pass keyword arguments. I did my best to keep this API private. I added functions prefixed with _Py: _PyObject_Fastcall(). Cython is eager to always use the fastest code and quickly adopted this new METH_FASTCALL calling convention... oops, I forgot to add a _Py prefix since these METH constants don't start with Py.

In 2019, this calling convention was extended to support also method calls (pass the self argument): PEP 590 – Vectorcall: a fast calling protocol for CPython. This new API is public and standardized. For example, it added public PyVectorcall_Function() and PyObject_Vectorcall() functions.

In 2023, the FASTCALL API is still around in the public C API:

• METH_FASTCALL
• _PyObject_FastCall()
• _PyObject_FastCallTstate()
• _PyObject_FastCallDict()
• _PyObject_FastCallDictTstate()
• _PyStack_AsDict()
• _PY_FASTCALL_SMALL_STACK

Can it be deprecated? Removed? I suppose that if we are in the unknown, the safe option is to start by deprecating it in Python 3.13 and plan its removal in Python 3.15.

cc @encukou

Victor

Linked PRs

• gh-106023: Update code using _PyObject_FastCall() #106257
• gh-106023: Remove _PY_FASTCALL_SMALL_STACK constant #106258
• gh-106023: Rename _PyObject_FastCallDictTstate() #106264
• gh-106023: Remove _PyObject_FastCall() function #106265
• gh-106023: Remove _PyObject_FastCallTstate() function #106273
• gh-106023: Update What's New in 3.13: _PyObject_FastCall() #117633
• Revert "gh-106023: Update What's New in 3.13: _PyObject_FastCall() (#117633)" #117676