Feature or enhancement

Some builtin functions accept None as a special value in addition to values that should have the specified type. For example start and stop indices in index(), size in read(), dir_fd in many os functions, and numerous string parameters, e.g. encoding and errors.

There are special format units in PyArg_Parse* functions for "string or None": z, z*, z#. There is half-public converter _PyEval_SliceIndex for indices, and several more private converters. But while NULL can non-ambiguously represent None in C, there is no single integer value for None. In many cases it is -1, but it is 0 and Py_SIZE(self) for indices, AT_FDCWD for dir_fd parameter, etc. This is why _PyEval_SliceIndex keeps the value of the C variable unchanged when the argument is None.

I propose to generalize it to all types. Add the ? modifier. If it occurs after the format unit, and the argument is None, it keeps the initial value of the C variable, like if the argument was optional and missed. For example:

    int dir_fd = AT_FDCWD;
    Py_ssize_t size = PY_SSIZE_T_MAX;
    double speed = 0.0;
    int sep = EOF;
    int flag = -1;
    if (!PyArg_ParseTuple(args, "i?n?d?C?p?", &dir_fd, &size, &speed, &sep, &flag)) {
        return NULL;
    }

The code accepts two integers, a float, a character, and an arbitrary object (as boolean), but any of 5 arguments can also be None, in which case the corresponding variable keeps its initial value.

There was a similar proposition for Argument Clinic: #64540. But this proposition works not only with Argument Clinic, therefore it can be used in third-party code. It works not only with integers, but with all types, including custom converters. It is more convenient, because do not need to use a special structure. I believe that it covers most of use cases, and in few remaining cases you still can handle it manually, as before.

After implementing this for PyArg_Parse* I will add support in Argument Clinic.

Linked PRs

• gh-112068: C API: Add support of nullable arguments in PyArg_Parse (prefix) #121187
• gh-112068: C API: Add support of nullable arguments in PyArg_Parse (suffix) #121303
• Revert "gh-112068: C API: Add support of nullable arguments in PyArg_Parse (GH-121303)" #136991
• [3.14] Revert "gh-112068: C API: Add support of nullable arguments in PyArg_Parse (GH-121303)" (GH-136991) #137006