Python 101: How to timeout a subprocess

The other day I ran into a use case where I needed to communicate with a subprocess I had started but I needed it to timeout. Unfortunately, Python 2 does not have a way to timeout the communicate method call so it just blocks until it either returns or the process itself closes. There are lots of different approaches that I found on StackOverflow, but I think my favorite was using Python's threading module's Timer class:

import subprocess

from threading import Timer

kill = lambda process: process.kill()
cmd = ['ping', 'www.google.com']
ping = subprocess.Popen(
    cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

my_timer = Timer(5, kill, [ping])

try:
    my_timer.start()
    stdout, stderr = ping.communicate()
finally:
    my_timer.cancel()

This particular example doesn't follow the use case I encountered exactly, but it's close. Basically what we have here is a long running process that we want to interact with. On Linux, if you call ping it will run indefinitely. So it makes good example. In this case, we write killing lambda that will call the process's kill method. Then we start the ping command, put it in a timer that's set to expire in 5 seconds and start the Timer. While the process runs, we collect its stdout and stderr and then the process dies. Finally we clean up by stopping the timer.

Python 3.5 added the run function which accepts a timeout parameter. According to the documentation, it will be passed to the subprocess's communicate method and TimeoutExpired exception will be raised should the process time out. Let's try it:

>>> import subprocess
>>> cmd = ['ping', 'www.google.com']
>>> subprocess.run(cmd, timeout=5)
PING www.google.com (216.58.216.196) 56(84) bytes of data.
64 bytes from ord31s21-in-f4.1e100.net (216.58.216.196): icmp_seq=1 ttl=55 time=16.3 ms
64 bytes from ord31s21-in-f4.1e100.net (216.58.216.196): icmp_seq=2 ttl=55 time=19.4 ms
64 bytes from ord31s21-in-f4.1e100.net (216.58.216.196): icmp_seq=3 ttl=55 time=20.0 ms
64 bytes from ord31s21-in-f4.1e100.net (216.58.216.196): icmp_seq=4 ttl=55 time=19.4 ms
64 bytes from ord31s21-in-f4.1e100.net (216.58.216.196): icmp_seq=5 ttl=55 time=17.0 ms
Traceback (most recent call last):
  Python Shell, prompt 3, line 1
  File "/usr/local/lib/python3.5/subprocess.py", line 711, in run
    stderr=stderr)
subprocess.TimeoutExpired: Command '['ping', 'www.google.com']' timed out after 5 seconds

It obviously worked the way it is documented. To be truly useful, we would probably want to wrap our subprocess call in an exception handler:

>>> try:
...     subprocess.run(cmd, timeout=5)
... except subprocess.TimeoutExpired:
...     print('process ran too long')
... 
PING www.google.com (216.58.216.196) 56(84) bytes of data.
64 bytes from ord31s21-in-f196.1e100.net (216.58.216.196): icmp_seq=1 ttl=55 time=18.3 ms
64 bytes from ord31s21-in-f196.1e100.net (216.58.216.196): icmp_seq=2 ttl=55 time=21.1 ms
64 bytes from ord31s21-in-f196.1e100.net (216.58.216.196): icmp_seq=3 ttl=55 time=22.7 ms
64 bytes from ord31s21-in-f196.1e100.net (216.58.216.196): icmp_seq=4 ttl=55 time=20.3 ms
64 bytes from ord31s21-in-f196.1e100.net (216.58.216.196): icmp_seq=5 ttl=55 time=16.8 ms
process ran too long

Now that we can catch the exception, we can continue doing something else or save the error exception. Interestingly enough, the timeout parameter was added to the subprocess module in Python 3.3. You can use it in subprocess.call, check_output, and check_call. It's also available in Popen.wait().