I have a Python script to capture network traffic with tcpdump in a subprocess:

p = subprocess.Popen(['tcpdump', '-I', '-i', 'en1','-w', 'cap.pcap'], stdout=subprocess.PIPE)
time.sleep(10)
p.kill()

When this script completes its work, I'm trying to open output .pcap file in Wireshark and getting this error:

"The capture file appears to have been cut short in the middle of a packet."

What solution could be applied for "proper" closing of tcpdump's subprocess?

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.