Python subprocess.call issue

Question

When I run the following in Python 3.2.3 in Linux it does nothing...

subprocess.call("export TZ=Australia/Adelaide", shell=True)

However if I run it in the terminal it works...

export TZ=Australia/Adelaide

I haven't had an issue with using subprocess.call before. Just seems to be this one. I'm running as a superuser so it's not a sudo thing, and I've also tried putting an r in front of the string to make it a raw string.

Any ideas? Thanks.

Answer 1

Export modifies the environment of the shell.

When you run it through subprocess, a new shell is created, the environment modified and then immediately destroyed.

When you run it in a shell, it modifies the environment of that shell so you can see the effect.

Answer 2

A subprocess (shell in this case) can't ( normally ) modify its parent environment.

To set the local timezone for the script and its children in Python (on Unix):

#!/usr/bin/env python3
import os
import time
from datetime import datetime, timezone

os.environ['TZ'] = 'Australia/Adelaide'
time.tzset()
print(datetime.now(timezone.utc).astimezone())
# -> 2015-09-25 05:02:52.784404+09:30

If you want to modify the environment for a single command then you could pass env parameter:

#!/usr/bin/env python
import os
import subprocess

subprocess.check_call('date', env=dict(os.environ, TZ='Australia/Adelaide'))
# -> Fri Sep 25 05:02:34 ACST 2015