How to control process environment variable in Python?

Question

I'm trying to change the environment of my Python execution process. It seems that the correct way to do that should be to interact with os.environ. However, I the following assertion fails:

import os, subprocess
os.environ['ATESTVARIABLE'] = 'value'
value = subprocess.check_output(['echo', '$ATESTVARIABLE'], shell=True)
assert 'value' in value

Is there something else that I should be doing to change the current environment? What flaw in my understanding of Python is revealed by the above code :)?

(Note that within the current Python interpreter, os.environ['ATESTVARIABLE'] contains the expected value. I am setting up to run some code which requires a specific environment variable, and which may launch external processes. Obviously, if I wanted to control the environment of a specific subprocess, I'd use the env keyword.)

Answer 1

Looking through the source code for the subprocess module, it's because using a list of arguments with shell=True will do the equivalent of...

/bin/sh -c 'echo' '$ATESTVARIABLE'

...when what you want is...

/bin/sh -c 'echo $ATESTVARIABLE'

The following works for me...

import os, subprocess

os.environ['ATESTVARIABLE'] = 'value'
value = subprocess.check_output('echo $ATESTVARIABLE', shell=True)
assert 'value' in value

---

Update

FWIW, the difference between the two is that the first form...

/bin/sh -c 'echo' '$ATESTVARIABLE'

...will just call the shell's built-in echo with no parameters, and set $0 to the literal string '$ATESTVARIABLE', for example...

$ /bin/sh -c 'echo $0'
/bin/sh
$ /bin/sh -c 'echo $0' '$ATESTVARIABLE'
$ATESTVARIABLE

...whereas the second form...

/bin/sh -c 'echo $ATESTVARIABLE'

...will call the shell's built-in echo with a single parameter equal to the value of the environment variable ATESTVARIABLE.