When a module import fails, other threads can receive a stale module reference that is no longer insys.modules.

Race condition

1. Thread 1 starts importing module X, adds it tosys.modules
2. Thread 2 checkssys.modules, finds module X, takes the "skip lock" fast path
3. Thread 1's import fails, removes module X fromsys.modules
4. Thread 2 returns the stale module reference (not insys.modules)

Affected code paths

• Lib/importlib/_bootstrap.py:_find_and_load() lines 1268-1282
• Python/import.c:PyImport_GetModule()
• Python/import.c:PyImport_ImportModuleLevelObject()

All three have an optimization that skips locking when a module appears already loaded, but none verify the module is still valid after checking_initializing.

Reproducer

importsysimportthreadingimporttempfileimportoswithtempfile.TemporaryDirectory()astmpdir:sys.path.insert(0,tmpdir)# Module that partially initializes then failswithopen(os.path.join(tmpdir,"failing_mod.py"),"w")asf:f.write("import time; time.sleep(0.05); raise RuntimeError('fail')")results= []defdo_import(delay):importtime;time.sleep(delay)try:mod=__import__("failing_mod")if"failing_mod"notinsys.modules:results.append("RACE: got module not in sys.modules")exceptRuntimeError:results.append("ok")t1=threading.Thread(target=do_import,args=(0,))t2=threading.Thread(target=do_import,args=(0.01,))t1.start();t2.start();t1.join();t2.join()print(results)# May show "RACE" on affected builds

Linked PRs

• gh-143650: Fix importlib race condition on import failure #143651
• [3.14] gh-143650: Fix importlib race condition on import failure (GH-143651) #144662
• [3.13] gh-143650: Fix importlib race condition on import failure (GH-143651) #144697