Broken circular import between `threading` and `_threading_local`

Bug report

Bug description:

The threading module defines local as follows:

https://github.com/python/cpython/blob/ee521e8ac19ad012ebc4e1b3e71b369988a9b9f8/Lib/threading.py#L67-L70

But _threading_local also uses threading as follows:

https://github.com/python/cpython/blob/ee521e8ac19ad012ebc4e1b3e71b369988a9b9f8/Lib/_threading_local.py#L120

This was fine at first, since threading never used thread-local data (the import was near the end). However, this changed in gh-114424:

https://github.com/python/cpython/blob/ee521e8ac19ad012ebc4e1b3e71b369988a9b9f8/Lib/threading.py#L1540-L1542

The _threading_local.local object depends on threading.current_thread(), and this dependency occurs when the object is created:

https://github.com/python/cpython/blob/ee521e8ac19ad012ebc4e1b3e71b369988a9b9f8/Lib/_threading_local.py#L93-L96
https://github.com/python/cpython/blob/ee521e8ac19ad012ebc4e1b3e71b369988a9b9f8/Lib/_threading_local.py#L42-L46

The object is created (and local is imported) before current_thread() is defined. As a result, there is the broken circular import, which can be reproduced as follows:

>>> import _thread
>>> del _thread._local
>>> import threading
Traceback (most recent call last):
  File "/home/user/.local/share/mise/installs/python/3.14.7/lib/python3.14/threading.py", line 67, in <module>
    from _thread import _local as local
ImportError: cannot import name '_local' from '_thread' (unknown location)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    import threading
  File "/home/user/.local/share/mise/installs/python/3.14.7/lib/python3.14/threading.py", line 69, in <module>
    from _threading_local import local
  File "/home/user/.local/share/mise/installs/python/3.14.7/lib/python3.14/_threading_local.py", line 120, in <module>
    from threading import current_thread, RLock
ImportError: cannot import name 'current_thread' from partially initialized module 'threading' (most likely due to a circular import) (/home/user/.local/share/mise/installs/python/3.14.7/lib/python3.14/threading.py)

Since the issue has not been reported before, this raises a legitimate question: is the fallback still relevant?

Related: #141460.

CPython versions tested on:

3.13, 3.14

Operating systems tested on:

Linux

Linked PRs

• gh-157042

Đọc Lib/threading.py và Lib/_threading_local.py tại các điểm import và tạo đối tượng được tham chiếu. Tái hiện lỗi bằng cách xóa _thread._local trước khi import threading, sau đó xem xét issue liên quan #141460 và PR được liên kết gh-157042. Được coi là hoàn thành khi import vòng lặp được giải quyết và hành vi fallback đã được minh họa được bao phủ bằng các bài kiểm thử hồi quy.