I’m in a research project that uses Python as the primary language for implementations and I’ve run into a very weird problem twice now. The issue is that some modules turn your code order sensitive for no apparent reason.

Here’s some snippets from a script that declares Celery tasks. TO ME, identical from a behavior standpoint. However, the first one doesn’t print anything. Why? I don’t know.

    @shared_task(bind=True)
    def start(self):
        print("Hello world")
        navegador = webdriver.Remote('http://firefox:4444/wd/hub',desired_capabilities=DesiredCapabilities.FIREFOX)
        # ...

    @shared_task(bind=True)
    def start(self):
        navegador = webdriver.Remote('http://firefox:4444/wd/hub',desired_capabilities=DesiredCapabilities.FIREFOX)
        print("Hello world")
        # ...

I ran into this issue earlier this year using OpenCV’s DNN module. In this case, running a subprocess after forwarding the network led to repeated results. I even posted a code to reproduce the problem in an issue on the opencv repo, but at the time of the writing this nobody has posted a definitive explanation or solution.

My guess is that this is related to how subprocess is implemented on Linux or perhaps how those subprocesses get started. It could also be a weird interaction between Python subprocesses and native extension threads (OpenCV makes use of OMP, TBB and plain threading).