I have a Python script to capture network traffic with tcpdumb in a subprocess:
p = subprocess.Popen(['tcpdump', '-I', '-i', 'en1',
'-w', 'cap.pcap'], stdout=subprocess.PIPE)
Instead of p.kill(), you can use p.send_signal(subprocess.signal.SIGTERM) to send a terminate signal rather than a kill (p.terminate() does the same)
The Popen docs describe the send_signal() command. The documentation on signals is a bit weak, but a dir(subprocess.signal) will list all the signals you may send to the process, but terminate should allow it some time to clean up.