How to find out when subprocess has terminated after using os.kill()?

I have a Python program (precisely, a Django application) that starts a subprocess using `subprocess.Popen`. Due to architecture constraints of my application, I'm not able to use `Popen.terminate()` to terminate the subprocess and `Popen.poll()` to check when the process has terminated. This is because I cannot hold a reference to the started subprocess in a variable. Instead, I have to write the process id `pid` to a file `pidfile` when the subprocess starts. When I want to stop the subprocess, I open this `pidfile` and use `os.kill(pid, signal.SIGTERM)` to stop it. My question is: How can I find out when the subprocess has really terminated? Using `signal.SIGTERM` it needs approximately 1-2 minutes to finally terminate after calling `os.kill()`. First I thought that `os.waitpid()` would be the right thing for this task but when I call it after `os.kill()` it gives me `OSError: [Errno 10] No child processes`. By the way, I'm starting and stopping the subprocess from a HTML template using two forms and the program logic is inside a Django view. The exception gets displayed in my browser when my application is in debug mode. It's probably also important to know that the subprocess that I call in my view (`python manage.py crawlwebpages`) itself calls another subprocess, namely an instance of a Scrapy crawler. I write the `pid` of this Scrapy instance to the `pidfile` and this is what I want to terminate. Here is the relevant code: ``` def process_main_page_forms(request): if request.method == 'POST': if request.POST['form-type'] == u'webpage-crawler-form': template_context = _crawl_webpage(request) elif request.POST['form-type'] == u'stop-crawler-form': template_context = _stop_crawler(request) else: template_context = { 'webpage_crawler_form': WebPageCrawlerForm(), 'stop_crawler_form': StopCrawlerForm()} return render(request, 'main.html', template_context) def _crawl_webpage(request): webpage_crawler_form = WebPageCrawlerForm(request.POST) if webpage_crawler_form.is_valid(): url_to_crawl = webpage_crawler_form.cleaned_data['url_to_crawl'] maximum_pages_to_crawl = webpage_crawler_form.cleaned_data['maximum_pages_to_crawl'] program = 'python manage.py crawlwebpages' + ' -n ' + str(maximum_pages_to_crawl) + ' ' + url_to_crawl p = subprocess.Popen(program.split()) template_context = { 'webpage_crawler_form': webpage_crawler_form, 'stop_crawler_form': StopCrawlerForm()} return template_context def _stop_crawler(request): stop_crawler_form = StopCrawlerForm(request.POST) if stop_crawler_form.is_valid(): with open('scrapy_crawler_process.pid', 'rb') as pidfile: process_id = int(pidfile.read().strip()) print 'PROCESS ID:', process_id os.kill(process_id, signal.SIGTERM) os.waitpid(process_id, os.WNOHANG) # This gives me the OSError print 'Crawler process terminated!' template_context = { 'webpage_crawler_form': WebPageCrawlerForm(), 'stop_crawler_form': stop_crawler_form} return template_context ``` What can I do? Thank you very much! EDIT: According to the great answer given by Jacek Konieczny, I could solve my problem by changing my code in the function `_stop_crawler(request)` to the following: ``` def _stop_crawler(request): stop_crawler_form = StopCrawlerForm(request.POST) if stop_crawler_form.is_valid(): with open('scrapy_crawler_process.pid', 'rb') as pidfile: process_id = int(pidfile.read().strip()) # These are the essential lines os.kill(process_id, signal.SIGTERM) while True: try: time.sleep(10) os.kill(process_id, 0) except OSError: break print 'Crawler process terminated!' template_context = { 'webpage_crawler_form': WebPageCrawlerForm(), 'stop_crawler_form': stop_crawler_form} return template_context ```