Python 线程加入死线程

Question

join()如果我调用一个已经完成的线程会发生什么？

例如

import threading
import time

def fn():
    time.sleep(1)

t = threading.Thread(target=fn)
t.start()

time.sleep(2)
t.join()

该文档似乎没有对这个问题提供任何说明

Answer 1

从您引用的文档中：

join：等待线程终止。...

因此，如果线程已经终止，当然它会立即退出。

文档的其他地方也有：

该操作将阻塞，直到线程终止。

好的，如果它已经终止，则操作不会阻塞。

该方法是一种在调用者和线程之间提供同步的方法。退出后join，保证线程结束。如果调用时线程已经结束join，那么它当然不会执行任何操作。

python 源代码证实了这一点（该函数是从以下位置调用的join()：

def _wait_for_tstate_lock(self, block=True, timeout=-1):
    # Issue #18808: wait for the thread state to be gone.
    # At the end of the thread's life, after all knowledge of the thread
    # is removed from C data structures, C code releases our _tstate_lock.
    # This method passes its arguments to _tstate_lock.acquire().
    # If the lock is acquired, the C code is done, and self._stop() is
    # called.  That sets ._is_stopped to True, and ._tstate_lock to None.
    lock = self._tstate_lock
    if lock is None:  # already determined that the C code is done
        assert self._is_stopped   # we see that we don't wait for anything here
    elif lock.acquire(block, timeout):
        lock.release()
        self._stop()