Hi, FYI I made a tiny enhancement in the subprocess module in Python 3.6: Popen destructor now logs a ResourceWarning if the child process is still running when the destructor is called. Keeping a child process in background is bad because it is likely to create zombi process, because Python will no more read its exit status. Martin Panter asked to add a new "detach() method" to subprocess.Popen if you would like to "transfer the ownership" of the child process (pid) to a different object. I'm not sure that it is needed in practice (Popen is responsible to handle different resources, not only the pid, maybe I didn't look closely to the issue), but it may be required for specific code to avoid the new ResourceWarning. Please take a look at : https://bugs.python.org/issue27068 Another enhancement: ResourceWarning now also logs where "leaked" resource was created if you enable the tracemalloc module. Example with tracemalloc logging 10 frames (truncated output for readability): --------------------- $ ./python -X tracemalloc=10 -m test test_sys test_os Run tests sequentially 0:00:00 [1/2] test_sys ^C .../Lib/subprocess.py:1023: ResourceWarning: subprocess 28856 is still running ResourceWarning, source=self) Object allocated at (most recent call first): File "/home/haypo/prog/python/default/Lib/test/test_sys.py", lineno 702 universal_newlines=True) (...) .../Lib/test/libregrtest/main.py:343: ResourceWarning: unclosed file <_io.TextIOWrapper name=3 encoding='UTF-8'> break Object allocated at (most recent call first): File "/home/haypo/prog/python/default/Lib/subprocess.py", lineno 955 self.stdout = io.TextIOWrapper(self.stdout) File "/home/haypo/prog/python/default/Lib/test/test_sys.py", lineno 702 universal_newlines=True) File "/home/haypo/prog/python/default/Lib/test/test_sys.py", lineno 714 out = self.c_locale_get_error_handler(encoding=':ignore') (...) Test suite interrupted by signal SIGINT. 2 tests omitted: test_os test_sys Total duration: 551 ms --------------------- Please run tests using -Wd (enabled by default in debug builds) and watch for ResourceWarning, and then report these warnings on the bug tracker. Each ResourceWarning is likely to highlight a bug. I just fixed the subprocess ResourceWarning in the test.support.script_helper module (the child process is now killed on error, and I added a context manager to cleanup all Popen resources). Victor