Enhance Argument Clinic's NoneType return converter to give void

[arhadthedev]

BPO	47128
Nosy	@larryhastings, @serhiy-storchaka, @arhadthedev
PRs	bpo-47128: Enhance Argument Clinic's NoneType return converter to give void #32126

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2022-03-27.06:26:32.947>
created_at = <Date 2022-03-26.12:59:28.016>
labels = ['type-feature', 'expert-argument-clinic', '3.11']
title = "Enhance Argument Clinic's NoneType return converter to give `void`"
updated_at = <Date 2022-03-28.07:34:15.520>
user = 'https://github.com/arhadthedev'

bugs.python.org fields:

activity = <Date 2022-03-28.07:34:15.520>
actor = 'arhadthedev'
assignee = 'none'
closed = True
closed_date = <Date 2022-03-27.06:26:32.947>
closer = 'serhiy.storchaka'
components = ['Argument Clinic']
creation = <Date 2022-03-26.12:59:28.016>
creator = 'arhadthedev'
dependencies = []
files = []
hgrepos = []
issue_num = 47128
keywords = ['patch']
message_count = 5.0
messages = ['416062', '416064', '416066', '416104', '416147']
nosy_count = 3.0
nosy_names = ['larry', 'serhiy.storchaka', 'arhadthedev']
pr_nums = ['32126']
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue47128'
versions = ['Python 3.11']

[arhadthedev]

The attached PR makes the following possible (note that the impl has a void return type):

/*[clinic input]
_io._IOBase.writelines -> NoneType
    lines: object
    /
[clinic start generated code]*/

    static void
    _io__IOBase_writelines_impl(PyObject *self, PyObject *lines)
    /*[clinic end generated code: output=f3feca36db72dbd1 input=286ba711cb7291ad]*/

Previously, the return type would be Object * with generated replacement of non-Py_None values to NULL on the other side.

So now there is no need to track whether NULL or Py_None should be returned. Or should it be Py_RETURN_NONE? Argument Clinic does it by itself returning NULL on errors and PyNone otherwise:

    static PyObject *
    _io__IOBase_writelines(PyObject *self, PyObject *lines)
    {
        PyObject *return_value = NULL;

        _io__IOBase_writelines_impl(self, lines);
        if (PyErr_Occurred()) {
            goto exit;
        }
        return_value = Py_None;
        Py_INCREF(Py_None);

    exit:
        return return_value;
    }

[serhiy-storchaka]

The function should return different values for success and error. Functions which do not do this have bad design.

[arhadthedev]

The function should return different values for success and error

It does, and a void return type enforces it. Here is the trick:

An important convention throughout the Python interpreter is the following: when a function fails, it should set an exception condition and return an error value (usually -1 or a NULL pointer).

• https://docs.python.org/3/extending/extending.html#intermezzo-errors-and-exceptions

Previously, a function could return NULL but forget to call PyErr_*(). With void, however, the function has no other choise but to properly set the error indicator so the Clinic's part will see it and return NULL finishing the convention.

[serhiy-storchaka]

It is simpler and faster to return NULL than call PyErr_Occurred(). There is a special macro PY_RETURN_NONE, so there is no problem with returning None either.

I do not think it would make the code better.

[arhadthedev]

Actually, you're right. For now, PyErr_Occurred is a GIL lock plus a memory access. While the access is cheap because of a L1 cache hit, the GIL takes its toll in a hot path.

So I'm closing the PR until GIL removal is done so no performance penalty will be imposed.

I could use _PyErr_Occurred because "Currently Argument Clinic is considered internal-only for CPython", but it requires extra modifications of the clinic that is undesirable.

[arhadthedev]

So I'm closing the PR until GIL removal is done so no performance penalty will be imposed.

Not relevant anymore; see #32126 (comment).