subprocess.check_output() doesn't close explicitly pipes if an error occurs. See for example issue #12493 for an example of an error on .communicate().

Attached patch uses a context manager to ensure that all pipes are always closed and that the status is read to avoid zombies.

Other subprocess functions should be fixed:
 - call() (will fix check_call)
 - getstatusoutput() (will fix getoutput): see patch attached to the issue #10197 to replace os.popen() by subprocess.Popen