gh-142737: Handle lost io.open in _Py_FindSourceFile

Lib/test/test_traceback.py

@@ -2268,7 +2268,7 @@ class TestTracebackFormat(unittest.TestCase, TracebackFormatMixin):
 
 @cpython_only
 @force_not_colorized_test_class
-class TestFallbackTracebackFormat(unittest.TestCase, TracebackFormatMixin):
+class TestFallbackFormatWhenPrinterRaises(unittest.TestCase, TracebackFormatMixin):
     DEBUG_RANGES = False
     def setUp(self) -> None:
         self.original_unraisable_hook = sys.unraisablehook
@@ -2282,6 +2282,34 @@ def tearDown(self) -> None:
         sys.unraisablehook = self.original_unraisable_hook
         return super().tearDown()
 
+@cpython_only
+@force_not_colorized_test_class
+class TestFallbackFormatWhenIOUnavailable(unittest.TestCase, TracebackFormatMixin):
+    def setUp(self) -> None:
+        import io
+        self.original_io = io
+        self.original_hook = traceback._print_exception_bltin
+        traceback._print_exception_bltin = object()
+        sys.modules['io'] = types.SimpleNamespace(StringIO=io.StringIO)
+        return super().setUp()
+
+    def test_unhashable(self):
+        raise unittest.SkipTest(
+            "test requires source lines to be captured in tracebacks")
+
+    def test_traceback_format_with_cleared_frames(self):
+        raise unittest.SkipTest(
+            "test requires source lines to be captured in tracebacks")
+
+    def test_traceback_format(self):
+        raise unittest.SkipTest(
+            "test requires source lines to be captured in tracebacks")
+
+    def tearDown(self) -> None:
+        sys.modules['io'] = self.original_io
+        traceback._print_exception_bltin = self.original_hook
+        return super().tearDown()
+
 class BaseExceptionReportingTests:
 
     def get_exception(self, exception_or_callable):

Objects/call.c

@@ -729,6 +729,9 @@ _Py_COMP_DIAG_POP
 PyObject * _PyObject_CallMethodFormat(PyThreadState *tstate, PyObject *callable,
                                       const char *format, ...)
 {
+    if (callable == NULL) {
+        return NULL;
+    }
     va_list va;
     va_start(va, format);
     PyObject *retval = callmethod(tstate, callable, format, va);