Python subprocess/Popen with a modified environmen

Question

I believe that running an external command with a slightly modified environment is a very common case. That's how I tend to do it:

import subprocess, os
my_env = os.environ
my_env["PATH"] = "/usr/sbin:/sbin:" + my_env["PATH"]
subprocess.Popen(my_command, env=my_env)

I've got a gut feeling that there's a better way; does it look alright?

Answer 1

I know this has been answered for some time, but there are some points that some may want to know about using PYTHONPATH instead of PATH in their environment variable. I have outlined an explanation of running python scripts with cronjobs that deals with the modified environment in a different way (found here). Thought it would be of some good for those who, like me, needed just a bit more than this answer provided.

Answer 2

With Python 3.5 you could do it this way:

import os
import subprocess

my_env = {**os.environ, 'PATH': '/usr/sbin:/sbin:' + os.environ['PATH']}

subprocess.Popen(my_command, env=my_env)

Here we end up with a copy of os.environ and overridden PATH value.

It was made possible by PEP 448 (Additional Unpacking Generalizations).

Another example. If you have a default environment (i.e. os.environ), and a dict you want to override defaults with, you can express it like this:

my_env = {**os.environ, **dict_with_env_variables}

Answer 3

The env parameter accepts a dictionary. You can simply take os.environ, add a key (your desired variable) (to a copy of the dict if you must) to that and use it as a parameter to Popen.

Answer 4

I think os.environ.copy() is better if you don't intend to modify the os.environ for the current process:

import subprocess, os
my_env = os.environ.copy()
my_env["PATH"] = "/usr/sbin:/sbin:" + my_env["PATH"]
subprocess.Popen(my_command, env=my_env)

Answer 5

you might use my_env.get("PATH", '') instead of my_env["PATH"] in case PATH somehow not defined in the original environment, but other than that it looks fine.

Answer 6

To temporarily set an environment variable without having to copy the os.envrion object etc, I do this:

process = subprocess.Popen(['env', 'RSYNC_PASSWORD=foobar', 'rsync', \
'rsync://[e-mail]::'], stdout=subprocess.PIPE)