Expose `threading.Semaphore._value`

Feature or enhancement

It would be useful to be able to access the value of threading.Semaphore._value as part of the public API of Semaphore, say via a get_value method.

Pitch

The value is useful for debugging and testing, helping users to understand the state and invariants of their code. There is also educational value in allowing users to see the internal state of a Semaphore, seeing as how it's an important concurrency primitive.

https://cs.github.com/?scopeName=All+repos&scope=&q=semaphore._value+language%3Apython# finds 68 uses of the exact phrase "semaphore._value" on Github, so I'm not alone in wanting access to this (although several of these are asyncio.locks.Semaphore).

Previous discussion

asyncio.locks.Semaphore has a locked method that exposes whether the value of the Semaphore is zero. It does appear that many of the uses are comparing the value to zero, so adding a locked method to threading.Semaphore would be sufficient for a lot of usage.

While writing this issue, I noticed that in 3.11, threading.Semaphore has its value printed in __repr__ (changed in #20534, the bpo mentions "expose their public states" as the rationale)

It looks like multiprocesing.Semaphore has an undocumented get_value method (although there is an issue about potentially removing it https://github.com/python/cpython/issues/84974 )

从 threading.Semaphore 及其当前的 repr 行为入手，然后比较 asyncio.locks.Semaphore.locked 和 multiprocessing.Semaphore.get_value。确定公共 API 是否应公开该值，还是仅公开其为零的状态，并在为所选接口定义测试和文档之前，考虑现有的讨论和理由。