How to evaluate environment variables into a string in Python?

Question

I have a string representing a path. Because this application is used on Windows, OSX and Linux, we’ve defined environment variables to properly map volumes from the different file systems. The result is:

"$C/test/testing"

What I want to do is evaluate the environment variables in the string so that they’re replaced by their respective volume names. Is there a specific command I’m missing, or do I have to take os.environ.keys() and manually replace the strings?

Asked By: Soviut

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Source

Answer 1

Use os.path.expandvars to expand the environment variables in the string, for example:

>>> os.path.expandvars('$C/test/testing')
'/stackoverflow/test/testing'

Answered By: jblocksom

Answer 2

In Python 3 you can do:

'{VAR}'.format(**os.environ))

for example

>>> 'hello from {PWD}'.format(**os.environ))
hello from /Users/william

Answered By: william_grisaitis

Answer 3

The os.path.expandvars solution will not expand variables that aren’t set.

If you want unset variables to be expanded to empty values (like the shell does), you can use string.Template, example:

import os
from string import Template
from collections import defaultdict

def expand_posix_vars(posix_expr, context):
    env = defaultdict(lambda: '')
    env.update(context)
    return Template(posix_expr).substitute(env)

conf_str = "${PREFIX}something"

assert "something" == expand_posix_vars(conf_str, {})
assert "hellosomething" == expand_posix_vars(conf_str, {"PREFIX": "hello"})

Answered By: Elias Dorneles

How to evaluate environment variables into a string in Python?

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