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 +2282,33 @@ 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(
+            "io unavailable (test requires source lines to be captured in tracebacks)")
+
+    def test_traceback_format_with_cleared_frames(self):
+        raise unittest.SkipTest(
+            "io unavailable (test requires source lines to be captured in tracebacks)")
+    def test_traceback_format(self):
+        raise unittest.SkipTest(
+            "io unavailable (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):

Misc/NEWS.d/next/Core_and_Builtins/2025-12-15-15-01-21.gh-issue-142737.xYXzeB.rst

@@ -0,0 +1,2 @@
+Tracebacks will be displayed in fallback mode even if :func:`io.open` is lost.
+Patch by Bartosz Sławecki.

Python/traceback.c

@@ -415,6 +415,9 @@ _Py_FindSourceFile(PyObject *filename, char* namebuf, size_t namelen, PyObject *
     npath = PyList_Size(syspath);
 
     open = PyObject_GetAttr(io, &_Py_ID(open));
+    if (open == NULL) {
+        goto error;
+    }
     for (i = 0; i < npath; i++) {
         v = PyList_GetItem(syspath, i);
         if (v == NULL) {