gh-105071: add PyUnstable_Exc_PrepReraiseStar to expose except* implementation in the unstable API (#105072)

Author: iritkatriel

Doc/c-api/exceptions.rst

@@ -772,6 +772,16 @@ Exception Objects
 
    Set :attr:`~BaseException.args` of exception *ex* to *args*.
 
+.. c:function:: PyObject* PyUnstable_Exc_PrepReraiseStar(PyObject *orig, PyObject *excs)
+
+   Implement part of the interpreter's implementation of :keyword:`!except*`.
+   *orig* is the original exception that was caught, and *excs* is the list of
+   the exceptions that need to be raised. This list contains the the unhandled
+   part of *orig*, if any, as well as the exceptions that were raised from the
+   :keyword:`!except*` clauses (so they have a different traceback from *orig*) and
+   those that were reraised (and have the same traceback as *orig*).
+   Return the :exc:`ExceptionGroup` that needs to be reraised in the end, or
+   ``None`` if there is nothing to reraise.
 
 .. _unicodeexceptions:
 

Include/cpython/pyerrors.h

@@ -116,6 +116,10 @@ PyAPI_FUNC(int) _PyException_AddNote(
      PyObject *exc,
      PyObject *note);
 
+PyAPI_FUNC(PyObject*) PyUnstable_Exc_PrepReraiseStar(
+     PyObject *orig,
+     PyObject *excs);
+
 /* In signalmodule.c */
 
 int PySignal_SetWakeupFd(int fd);

Lib/test/test_capi/test_exceptions.py

@@ -5,6 +5,7 @@
 from test import support
 from test.support import import_helper
 from test.support.script_helper import assert_python_failure
+from test.support.testcase import ExceptionIsLikeMixin
 
 from .test_misc import decode_stderr
 
@@ -189,5 +190,97 @@ def __repr__(self):
                          'Normalization failed: type=Broken args=<unknown>')
 
 
+class Test_PyUnstable_Exc_PrepReraiseStar(ExceptionIsLikeMixin, unittest.TestCase):
+
+    def setUp(self):
+        super().setUp()
+        try:
+            raise ExceptionGroup("eg", [TypeError('bad type'), ValueError(42)])
+        except ExceptionGroup as e:
+            self.orig = e
+
+    def test_invalid_args(self):
+        with self.assertRaisesRegex(TypeError, "orig must be an exception"):
+            _testcapi.unstable_exc_prep_reraise_star(42, [None])
+
+        with self.assertRaisesRegex(TypeError, "excs must be a list"):
+            _testcapi.unstable_exc_prep_reraise_star(self.orig, 42)
+
+        with self.assertRaisesRegex(TypeError, "not an exception"):
+            _testcapi.unstable_exc_prep_reraise_star(self.orig, [TypeError(42), 42])
+
+        with self.assertRaisesRegex(ValueError, "orig must be a raised exception"):
+            _testcapi.unstable_exc_prep_reraise_star(ValueError(42), [TypeError(42)])
+
+        with self.assertRaisesRegex(ValueError, "orig must be a raised exception"):
+            _testcapi.unstable_exc_prep_reraise_star(ExceptionGroup("eg", [ValueError(42)]),
+                                                     [TypeError(42)])
+
+
+    def test_nothing_to_reraise(self):
+        self.assertEqual(
+            _testcapi.unstable_exc_prep_reraise_star(self.orig, [None]), None)
+
+        try:
+            raise ValueError(42)
+        except ValueError as e:
+            orig = e
+        self.assertEqual(
+            _testcapi.unstable_exc_prep_reraise_star(orig, [None]), None)
+
+    def test_reraise_orig(self):
+        orig = self.orig
+        res = _testcapi.unstable_exc_prep_reraise_star(orig, [orig])
+        self.assertExceptionIsLike(res, orig)
+
+    def test_raise_orig_parts(self):
+        orig = self.orig
+        match, rest = orig.split(TypeError)
+
+        test_cases = [
+            ([match, rest], orig),
+            ([rest, match], orig),
+            ([match], match),
+            ([rest], rest),
+            ([], None),
+        ]
+
+        for input, expected in test_cases:
+            with self.subTest(input=input):
+                res = _testcapi.unstable_exc_prep_reraise_star(orig, input)
+                self.assertExceptionIsLike(res, expected)
+
+
+    def test_raise_with_new_exceptions(self):
+        orig = self.orig
+
+        match, rest = orig.split(TypeError)
+        new1 = OSError('bad file')
+        new2 = RuntimeError('bad runtime')
+
+        test_cases = [
+            ([new1, match, rest], ExceptionGroup("", [new1, orig])),
+            ([match, new1, rest], ExceptionGroup("", [new1, orig])),
+            ([match, rest, new1], ExceptionGroup("", [new1, orig])),
+
+            ([new1, new2, match, rest], ExceptionGroup("", [new1, new2, orig])),
+            ([new1, match, new2, rest], ExceptionGroup("", [new1, new2, orig])),
+            ([new2, rest, match, new1], ExceptionGroup("", [new2, new1, orig])),
+            ([rest, new2, match, new1], ExceptionGroup("", [new2, new1, orig])),
+
+
+            ([new1, new2, rest], ExceptionGroup("", [new1, new2, rest])),
+            ([new1, match, new2], ExceptionGroup("", [new1, new2, match])),
+            ([rest, new2, new1], ExceptionGroup("", [new2, new1, rest])),
+            ([new1, new2], ExceptionGroup("", [new1, new2])),
+            ([new2, new1], ExceptionGroup("", [new2, new1])),
+        ]
+
+        for (input, expected) in test_cases:
+            with self.subTest(input=input):
+                res = _testcapi.unstable_exc_prep_reraise_star(orig, input)
+                self.assertExceptionIsLike(res, expected)
+
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/C API/2023-05-30-10-15-13.gh-issue-105071.dPtp7c.rst

@@ -0,0 +1 @@
+Add ``PyUnstable_Exc_PrepReraiseStar`` to the unstable C api to expose the implementation of :keyword:`except* <except_star>`.

Modules/_testcapi/clinic/exceptions.c.h

@@ -288,6 +288,22 @@ _testcapi_traceback_print_impl(PyObject *module, PyObject *traceback,
     Py_RETURN_NONE;
 }
 
+/*[clinic input]
+_testcapi.unstable_exc_prep_reraise_star
+    orig: object
+    excs: object
+    /
+To test PyUnstable_Exc_PrepReraiseStar.
+[clinic start generated code]*/
+
+static PyObject *
+_testcapi_unstable_exc_prep_reraise_star_impl(PyObject *module,
+                                              PyObject *orig, PyObject *excs)
+/*[clinic end generated code: output=850cf008e0563c77 input=27fbcda2203eb301]*/
+{
+    return PyUnstable_Exc_PrepReraiseStar(orig, excs);
+}
+
 
 /*
  * Define the PyRecurdingInfinitelyError_Type
@@ -328,6 +344,7 @@ static PyMethodDef test_methods[] = {
     _TESTCAPI_SET_EXCEPTION_METHODDEF
     _TESTCAPI_TRACEBACK_PRINT_METHODDEF
     _TESTCAPI_WRITE_UNRAISABLE_EXC_METHODDEF
+    _TESTCAPI_UNSTABLE_EXC_PREP_RERAISE_STAR_METHODDEF
     {NULL},
 };
 

Modules/_testcapi/exceptions.c

@@ -1351,7 +1351,10 @@ is_same_exception_metadata(PyObject *exc1, PyObject *exc2)
 PyObject *
 _PyExc_PrepReraiseStar(PyObject *orig, PyObject *excs)
 {
+    /* orig must be a raised & caught exception, so it has a traceback */
     assert(PyExceptionInstance_Check(orig));
+    assert(_PyBaseExceptionObject_cast(orig)->traceback != NULL);
+
     assert(PyList_Check(excs));
 
     Py_ssize_t numexcs = PyList_GET_SIZE(excs);
@@ -1438,6 +1441,42 @@ _PyExc_PrepReraiseStar(PyObject *orig, PyObject *excs)
     return result;
 }
 
+PyObject *
+PyUnstable_Exc_PrepReraiseStar(PyObject *orig, PyObject *excs)
+{
+    if (orig == NULL || !PyExceptionInstance_Check(orig)) {
+        PyErr_SetString(PyExc_TypeError, "orig must be an exception instance");
+        return NULL;
+    }
+    if (excs == NULL || !PyList_Check(excs)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "excs must be a list of exception instances");
+        return NULL;
+    }
+    Py_ssize_t numexcs = PyList_GET_SIZE(excs);
+    for (Py_ssize_t i = 0; i < numexcs; i++) {
+        PyObject *exc = PyList_GET_ITEM(excs, i);
+        if (exc == NULL || !(PyExceptionInstance_Check(exc) || Py_IsNone(exc))) {
+            PyErr_Format(PyExc_TypeError,
+                         "item %d of excs is not an exception", i);
+            return NULL;
+        }
+    }
+
+    /* Make sure that orig has something as traceback, in the interpreter
+     * it always does becuase it's a raised exception.
+     */
+    PyObject *tb = PyException_GetTraceback(orig);
+
+    if (tb == NULL) {
+        PyErr_Format(PyExc_ValueError, "orig must be a raised exception");
+        return NULL;
+    }
+    Py_DECREF(tb);
+
+    return _PyExc_PrepReraiseStar(orig, excs);
+}
+
 static PyMemberDef BaseExceptionGroup_members[] = {
     {"message", T_OBJECT, offsetof(PyBaseExceptionGroupObject, msg), READONLY,
         PyDoc_STR("exception message")},