The subprocess call *subprocess.Popen* in Python3.6 was added a number of resource warnings, including subprocess run-state and open files. This is a very good facility for debugging, but causes a lot of trouble for programs relying on subprocesses via the STDIO/STDERR interface. The STDIO/STDERR interfaces are very common when shell utilities are incorporated into high level Python programs.

The other issue is the unit testing of command line tools as black-box tests, these solely rely on the STDOUT and STDERR interface. I am currently finishing a subprocess test package with common code for Python2.7 and Python3.5+, so facing some trouble with IO filtering. Examples are attached.

A system call should process the common IO interfaces of the called subprocesses by default without any additional output. So a call flag and/or an environment variable should be introduced in addition, which allows the activation and deactivation of these messages.
The default should be *deactive*.

See also 
issue26741 - subprocess.Popen should emit a ResourceWarning in destructor if ...

Can’t you use Python’s existing CLI <https://docs.python.org/3.6/using/cmdline.html#cmdoption-w> and environment variable <https://docs.python.org/3.6/using/cmdline.html#envvar-PYTHONWARNINGS> to control the ResourceWarning messages?

Warnings where enabled by default when using the “unittest” module in Issue 10535. There is probably a way to disable them, at least by using the “warnings” module directly. But I think ResourceWarning should stay enabled by default when running tests. As you said, the warnings help testing for bugs.