线程问题

想问一个问题，既然 threading 模块里面 join 的官方解释是： join([timeout]) Wait until the thread terminates. This blocks the calling thread until the thread whose join() method is called terminates – either normally or through an unhandled exception – or until the optional timeout occurs.

When the timeout argument is present and not None, it should be a floating point number specifying a timeout for the operation in seconds (or fractions thereof). As join() always returns None, you must call isAlive() after join() to decide whether a timeout happened – if the thread is still alive, the join() call timed out.

那它是怎样去对 threading 是否 termaine 进行判断吗？还是 python 通过信号量来进行判断的，求大神解释