Process stops logging after last `expect`

[pexpect]

Process stops logging after last `expect`

[takluyver]

Reading data is a 'pull' operation, not a 'push' - the parent process has to explicitly call something to read data from the child. You can't reassign file descriptors of another process - e.g. to connect stdout directly to a file. You might want to spin out a separate thread (or use the asyncio event loop) to pull data from it and store it somewhere.

[jquast]

Right, this is by design. Though you're not the first to ask, so it probably begs the need to document about this somewhere in the docs. I'm going to leave this open for such purpose.

I second @takluyver's recommendation -- spawn a thread that keeps calling readline() or read_nonblocking(). Your generate.sh will block at some later point when its stdout buffer is filled unless you begin to expect or read it. See #54 (comment)

If you wanted to redirect stdout to your main program, it could be as simple as:

class ContinueReadPrintLoop(threading.Thread):
    def __init__(self, child):
        self.child = child
        threading.Thread.__init__(self)

    def run(self):
        while True:
            try:
                print(self.child.readline().rstrip())
            except pexpect.EOF:
                print('EOF')
                break

child = pexpect.spawn('....')
child.expect(['this', 'and', 'that'])
ContinueReadPrintLoop(child).start()

(you would also need to use a shared threading.Lock instance acquire a lock to print anything to stdout if your main thread also prints, or if you run several of these children)

and yet, then what would you do? You wouldn't be able to interact with child anymore, is it OK if you don't know or care when or how it exits? Maybe what you really want is a background thread that just does child.expect(pexpect.EOF), at least this way, you can still be assured of the exit status of the child process.

[takluyver]

I suspect if we integrate this in Pexpect, it should also store it in self.buffer, in case you later want to expect something in it. Then there's some complexity about doing it with threading, because you need to ensure it stops before something else tries to read .buffer.

[takluyver]

Sorry, half asleep, I meant "if we integrate this in Pexpect"

[jquast]

Marking as duplicate of #54. Otherwise submit a PR for a self-answering FAQ entry to reduce the misunderstanding of expect pull operation