Defensive programming for unhandled errors. Add this to preexec_fn= in your subprocess calls.

ensure_dead_with_parent.py

import ctypes
import sys
import signal

def ensure_dead_with_parent():
    """A last resort measure to make sure this process dies with its parent.
    Defensive programming for unhandled errors.
    """
    if not sys.platform.startswith('linux'):
        return  # not supported on OS X, Windows, etc.  Use process groups.
    PR_SET_PDEATHSIG = 1  # include/uapi/linux/prctl.h
    libc = ctypes.CDLL(ctypes.util.find_library('c'))
    libc.prctl(PR_SET_PDEATHSIG, signal.SIGKILL)