Popen.poll() doesn't reflect the returncode value when using a callback

[inorton]

I have a number of callers that do..

proc = subprocess.Popen(cmdline)
while proc.poll() is None:
    time.sleep(1)

I register with:

def my_callback():
    print("called")
    time.sleep(1)

fp.register(cmdline, returncode=1, callback=my_callback)

But proc.poll() seems to always return None

[aklajnert]

The returncode is ignored when passing callback, as the callback was meant to modify the returncode like in the example below:

def test_poll(fp):
    def my_callback(proc):
        print("called")
        time.sleep(1)
        proc.returncode = 1

    fp.register("test", returncode=1, callback=my_callback)

    proc = subprocess.Popen("test")
    while proc.poll() is None:
        time.sleep(1)

This is a bit counterintuitive, but I'm not sure how it is supposed to be fixed. I see two options:

• just raise an exception when callback and returncode are provided together, and inform users that the returncode shall be set within a callback,
• set the returncode if it was never modified by a callback

The second option seems to be better, but it could confuse people that are changing the returncode within the callback, but still expect it to finally land on the original returncode provided in the fp.register().

[inorton]

Oh I see, I must have missed that. It would be good perhaps to provide a type hint for the callback?

[aklajnert]

I'll think about it. It may be confusing also for other, so some warning or even error in such cases could be beneficial. I'll keep this issue open until I'll figure something out.