Currently, `test_pycfunction` picks a few built-in functions with various calling conventions to exercise all the relevant code paths:

        for py_name, py_args, c_name, expected_frame_number in (
            ('gmtime', '', 'time_gmtime', 1),  # METH_VARARGS
            ('len', '[]', 'builtin_len', 2),  # METH_O
            ...

See: https://github.com/python/cpython/blob/2f19e82fbe98ce86bcd98a176328af2808b678e8/Lib/test/test_gdb.py#L851

These calling conventions are not a guaranteed part of the interface, and as such these tests are fragile (as we saw in GH-14311, when converting the time module to use Argument Clinic changed gmtime from METH_VARARGS to METH_FASTCALL). Per Victor's suggestion in GH-14330, I think we should expose a few test functions in the `testcapi` module as exemplars of their respective calling conventions and use those, rather than arbitrary builtins.

> Per Victor's suggestion in GH-14330, I think we should expose a few test functions in the `testcapi` module as exemplars of their respective calling conventions and use those, rather than arbitrary builtins.

I suggest to write functions which return their arguments, so we can check that we get what we expect ;-)

By the way, PyObject_CallFunction("O") has a surprising behavior: if you pass a tuple, the tuple is unpacked, whereas any other type is passed directly. See bpo-28977.

I would prefer to fix the API since it caused severe bugs in the past, like asyncio generator bug bpo-21209.

But I'm not sure if we can do it without a deprecation period... I'm not sure that we should deprecate anything, since I would expect that PyObject_CallFunction(func, "O", tuple) calls func(tuple) rather than func(*tuple)...

New changeset f958377b67c36a98d4df67b94c01eb29e3104f61 by Miss Islington (bot) (Petr Viktorin) in branch 'master':
bpo-37499: Test various C calling conventions (GH-15776)
https://github.com/python/cpython/commit/f958377b67c36a98d4df67b94c01eb29e3104f61

This is done, thanks Petr and Jeroen!

I don't see a need to backport this unless we also want to backport GH-14311 or something else that depends on it.