Can isAlive() be False immediately after calling start() because the thread hasn't yet started?

Question

In the python documentation at http://docs.python.org/2/library/threading.html#thread-objects it says that

[isAlive()] returns True just before the run() method starts until just after the run() method terminates

But then the start() method says that:

[start()] arranges for the object’s run() method to be invoked in a separate thread of control.

Does this mean if I call t.start() and then immediately check t.isAlive() it's possible I could get False because the thread hasn't started yet?

Answer 1

It can't happen, at least not in CPython's implementation. That comes from staring at the code for Thread.start (here from the Python 3 source, but it doesn't matter):

def start(self):
    ...
    try:
        _start_new_thread(self._bootstrap, ())
    except Exception:
        with _active_limbo_lock:
            del _limbo[self]
        raise
    self._started.wait()

_start_new_thread() is implemented in C, starting a new thread and running self._bootstrap() inside that new thread. self._bootstrap() in turn invokes self.run(). If that's all there were to it, then the invoking thread could indeed return an arbitrary amount of time before run() started to execute. But the:

    self._started.wait()

at the end blocks on an internal Event. The bootstrap code sets the _started Event shortly before invoking run(), and the state of that same event is the primary thing isAlive() looks at.