gh-67377: Document that PyErr_SetString, etc. chain exceptions

[cjerdonek]

• Issue: Provide a C helper function to chain raised (but not yet caught) exceptions #67377

[serhiy-storchaka]

It is not clear what "caught" mean. The rough equivalent of the Python code:

try:
    ...
except AttributeError:
    raises TypeError

is the C code:

...
if (result == NULL) {
    if (PyErr_ExceptionMatches(PyExc_AttributeError)) {
        PyErr_SetNone(PyExc_TypeError);
    }
    return NULL;
}

Is an AttributeError exception caught here?

[cjerdonek]

The precise condition inside _PyErr_SetObject is if _PyErr_GetTopmostException returns a non-None value:

cpython/Python/errors.c

Lines 117 to 119 in be63019

	exc_value = _PyErr_GetTopmostException(tstate)->exc_value;
	if (exc_value != NULL && exc_value != Py_None) {
	/* Implicit exception chaining */

However, this is a private function.

So some other ways to say this (without reference to C functions) are (1) if there's an "active exception" (the wording in do_raise()'s RuntimeError message), (2) if an exception is currently being handled (anywhere on the stack), or (3) if sys.exc_info() would return an exception.

sys.exc_info()'s documentation
https://docs.python.org/3/library/sys.html#sys.exc_info

Do you have a preference?

[JelleZijlstra]

I would say "active exception". Could you make that change? In that case, I can merge this PR unless @serhiy-storchaka has further comments.

[serhiy-storchaka]

Is the term "active exception" explained anywhere? Should not a link be added?

[JelleZijlstra]

The term used by the PyErr_Occurred docs is "the error indicator is set".

[furkanonder]

LGTM. @serhiy-storchaka Can you review it?

[iritkatriel]

@serhiy-storchaka raised an issue with this terminology, I don't think the previous reviewers' comments have been addressed: #20329 (comment)

[python-cla-bot]

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