gh-136517: Print uncollectable objects if DEBUG_UNCOLLECTABLE mode was set

Lib/test/test_gc.py

@@ -732,13 +732,18 @@ def run_command(code):
         self.assertIn(b"ResourceWarning: gc: 2 uncollectable objects at "
                       b"shutdown; use", stderr)
         self.assertNotIn(b"<X 'first'>", stderr)
+        one_line_re = b"gc: uncollectable <X 0x[0-9A-F]+>"
+        expected_re = one_line_re + b"\\r?\\n" + one_line_re
+        self.assertNotRegex(stderr, expected_re)
         # With DEBUG_UNCOLLECTABLE, the garbage list gets printed
         stderr = run_command(code % "gc.DEBUG_UNCOLLECTABLE")
         self.assertIn(b"ResourceWarning: gc: 2 uncollectable objects at "
                       b"shutdown", stderr)
         self.assertTrue(
             (b"[<X 'first'>, <X 'second'>]" in stderr) or
             (b"[<X 'second'>, <X 'first'>]" in stderr), stderr)
+        # we expect two lines with uncollectable objects
+        self.assertRegex(stderr, expected_re)
         # With DEBUG_SAVEALL, no additional message should get printed
         # (because gc.garbage also contains normally reclaimable cyclic
         # references, and its elements get printed at runtime anyway).

Misc/NEWS.d/next/Core_and_Builtins/2025-07-10-23-23-50.gh-issue-136517._NHJyv.rst

@@ -0,0 +1,2 @@
+Fixed a typo that prevented printing of uncollectable objects when the
+DEBUG_UNCOLLECTABLE mode was set

Python/gc.c

@@ -1782,7 +1782,7 @@ gc_collect_region(PyThreadState *tstate,
     Py_ssize_t n = 0;
     for (gc = GC_NEXT(&finalizers); gc != &finalizers; gc = GC_NEXT(gc)) {
         n++;
-        if (gcstate->debug & _PyGC_DEBUG_COLLECTABLE)
+        if (gcstate->debug & _PyGC_DEBUG_UNCOLLECTABLE)
             debug_cycle("uncollectable", FROM_GC(gc));
     }
     stats->uncollectable = n;