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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"importlib race condition allows stale module ref to escape on failure","articleBody":"When a module import fails, other threads can receive a stale module reference that is no longer in `sys.modules`.\n\n## Race condition\n\n1. Thread 1 starts importing module X, adds it to `sys.modules`\n2. Thread 2 checks `sys.modules`, finds module X, takes the \"skip lock\" fast path\n3. Thread 1's import fails, removes module X from `sys.modules`\n4. Thread 2 returns the stale module reference (not in `sys.modules`)\n\n## Affected code paths\n\n- `Lib/importlib/_bootstrap.py:_find_and_load()` lines 1268-1282\n- `Python/import.c:PyImport_GetModule()` \n- `Python/import.c:PyImport_ImportModuleLevelObject()`\n\nAll three have an optimization that skips locking when a module appears already loaded, but none verify the module is still valid after checking `_initializing`.\n\n## Reproducer\n\n```python\nimport sys\nimport threading\nimport tempfile\nimport os\n\nwith tempfile.TemporaryDirectory() as tmpdir:\n    sys.path.insert(0, tmpdir)\n    \n    # Module that partially initializes then fails\n    with open(os.path.join(tmpdir, \"failing_mod.py\"), \"w\") as f:\n        f.write(\"import time; time.sleep(0.05); raise RuntimeError('fail')\")\n    \n    results = []\n    def do_import(delay):\n        import time; time.sleep(delay)\n        try:\n            mod = __import__(\"failing_mod\")\n            if \"failing_mod\" not in sys.modules:\n                results.append(\"RACE: got module not in sys.modules\")\n        except RuntimeError:\n            results.append(\"ok\")\n    \n    t1 = threading.Thread(target=do_import, args=(0,))\n    t2 = threading.Thread(target=do_import, args=(0.01,))\n    t1.start(); t2.start(); t1.join(); t2.join()\n    print(results)  # May show \"RACE\" on affected builds\n```\n\n\u003c!-- gh-linked-prs -->\n### Linked PRs\n* gh-143651\n* gh-144662\n* gh-144697\n\u003c!-- /gh-linked-prs -->\n","author":{"url":"https://github.com/gpshead","@type":"Person","name":"gpshead"},"datePublished":"2026-01-10T10:57:32Z","url":"https://github.com/python/cpython/issues/143650"}