[ Python 常見問題 ] How to evaluate environment variables into a string in Python?

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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:

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1. "$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?

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

>>> os.environ['VOLUME_PATH']='/tmp'
>>> os.path.expandvars('$VOLUME_PATH/abc')
'/tmp/abc'
>>> os.environ['VOLUME_PATH']='/tmp/test'
>>> os.path.expandvars('$VOLUME_PATH/abc')
'/tmp/test/abc'