gh-107782: Use _testcapi to test non-representable signatures (GH-109325)

Builtin functions and methods that have non-representable signatures today
will have representable signatures yesterday, and they will become unusable
for testing this feature.

So we need to add special functions and methods to the _testcapi module
that always have non-representable signatures.

Lib/test/test_pydoc.py

@@ -24,6 +24,7 @@ import textwrap
 from io import StringIO
 from collections import namedtuple
 from urllib.request import urlopen, urlcleanup
+from test import support
 from test.support import import_helper
 from test.support import os_helper
 from test.support.script_helper import (assert_python_ok,
@@ -1236,22 +1237,56 @@ class TestDescriptions(unittest.TestCase):
         self.assertEqual(self._get_summary_line(dict.__class_getitem__),
             "__class_getitem__(object, /) method of builtins.type instance")
 
+    @support.cpython_only
     def test_module_level_callable_unrepresentable_default(self):
-        self.assertEqual(self._get_summary_line(getattr),
-            "getattr(...)")
+        import _testcapi
+        builtin = _testcapi.func_with_unrepresentable_signature
+        self.assertEqual(self._get_summary_line(builtin),
+            "func_with_unrepresentable_signature(a, b=<x>)")
 
+    @support.cpython_only
     def test_builtin_staticmethod_unrepresentable_default(self):
         self.assertEqual(self._get_summary_line(str.maketrans),
             "maketrans(x, y=<unrepresentable>, z=<unrepresentable>, /)")
+        import _testcapi
+        cls = _testcapi.DocStringUnrepresentableSignatureTest
+        self.assertEqual(self._get_summary_line(cls.staticmeth),
+            "staticmeth(a, b=<x>)")
 
+    @support.cpython_only
     def test_unbound_builtin_method_unrepresentable_default(self):
         self.assertEqual(self._get_summary_line(dict.pop),
             "pop(self, key, default=<unrepresentable>, /)")
+        import _testcapi
+        cls = _testcapi.DocStringUnrepresentableSignatureTest
+        self.assertEqual(self._get_summary_line(cls.meth),
+            "meth(self, /, a, b=<x>)")
 
+    @support.cpython_only
     def test_bound_builtin_method_unrepresentable_default(self):
         self.assertEqual(self._get_summary_line({}.pop),
             "pop(key, default=<unrepresentable>, /) "
             "method of builtins.dict instance")
+        import _testcapi
+        obj = _testcapi.DocStringUnrepresentableSignatureTest()
+        self.assertEqual(self._get_summary_line(obj.meth),
+            "meth(a, b=<x>) "
+            "method of _testcapi.DocStringUnrepresentableSignatureTest instance")
+
+    @support.cpython_only
+    def test_unbound_builtin_classmethod_unrepresentable_default(self):
+        import _testcapi
+        cls = _testcapi.DocStringUnrepresentableSignatureTest
+        descr = cls.__dict__['classmeth']
+        self.assertEqual(self._get_summary_line(descr),
+            "classmeth(type, /, a, b=<x>)")
+
+    @support.cpython_only
+    def test_bound_builtin_classmethod_unrepresentable_default(self):
+        import _testcapi
+        cls = _testcapi.DocStringUnrepresentableSignatureTest
+        self.assertEqual(self._get_summary_line(cls.classmeth),
+            "classmeth(a, b=<x>) method of builtins.type instance")
 
     def test_overridden_text_signature(self):
         class C:

Modules/_testcapi/docstring.c

@@ -100,6 +100,13 @@ static PyMethodDef test_methods[] = {
     {"test_with_docstring",
         test_with_docstring,              METH_VARARGS,
         PyDoc_STR("This is a pretty normal docstring.")},
+    {"func_with_unrepresentable_signature",
+        (PyCFunction)test_with_docstring, METH_VARARGS,
+        PyDoc_STR(
+            "func_with_unrepresentable_signature($module, /, a, b=<x>)\n"
+            "--\n\n"
+            "This docstring has a signature with unrepresentable default."
+        )},
     {NULL},
 };
 
@@ -140,6 +147,40 @@ static PyTypeObject DocStringNoSignatureTest = {
     .tp_new = PyType_GenericNew,
 };
 
+static PyMethodDef DocStringUnrepresentableSignatureTest_methods[] = {
+    {"meth",
+        (PyCFunction)test_with_docstring, METH_VARARGS,
+        PyDoc_STR(
+            "meth($self, /, a, b=<x>)\n"
+            "--\n\n"
+            "This docstring has a signature with unrepresentable default."
+        )},
+    {"classmeth",
+        (PyCFunction)test_with_docstring, METH_VARARGS|METH_CLASS,
+        PyDoc_STR(
+            "classmeth($type, /, a, b=<x>)\n"
+            "--\n\n"
+            "This docstring has a signature with unrepresentable default."
+        )},
+    {"staticmeth",
+        (PyCFunction)test_with_docstring, METH_VARARGS|METH_STATIC,
+        PyDoc_STR(
+            "staticmeth(a, b=<x>)\n"
+            "--\n\n"
+            "This docstring has a signature with unrepresentable default."
+        )},
+    {NULL},
+};
+
+static PyTypeObject DocStringUnrepresentableSignatureTest = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    .tp_name = "_testcapi.DocStringUnrepresentableSignatureTest",
+    .tp_basicsize = sizeof(PyObject),
+    .tp_flags = Py_TPFLAGS_DEFAULT,
+    .tp_methods = DocStringUnrepresentableSignatureTest_methods,
+    .tp_new = PyType_GenericNew,
+};
+
 int
 _PyTestCapi_Init_Docstring(PyObject *mod)
 {
@@ -149,5 +190,8 @@ _PyTestCapi_Init_Docstring(PyObject *mod)
     if (PyModule_AddType(mod, &DocStringNoSignatureTest) < 0) {
         return -1;
     }
+    if (PyModule_AddType(mod, &DocStringUnrepresentableSignatureTest) < 0) {
+        return -1;
+    }
     return 0;
 }

Tools/c-analyzer/cpython/ignored.tsv

@@ -420,6 +420,7 @@ Modules/_testcapi/buffer.c	-	testBufType	-
 Modules/_testcapi/code.c	get_code_extra_index	key	-
 Modules/_testcapi/datetime.c	-	test_run_counter	-
 Modules/_testcapi/docstring.c	-	DocStringNoSignatureTest	-
+Modules/_testcapi/docstring.c	-	DocStringUnrepresentableSignatureTest	-
 Modules/_testcapi/exceptions.c	-	PyRecursingInfinitelyError_Type	-
 Modules/_testcapi/heaptype.c	-	_testcapimodule	-
 Modules/_testcapi/mem.c	-	FmData	-