I've several times wanted to expand a string in the way that os.path.expandvars does, but without modifying the actual environment.

If for example I have 

inputString = "$ROOT/${SUBDIR}/some/path"

and I want to find expanded versions of this, I have to do something like

os.environ["ROOT"] = "/users/geoff"
os.environ["SUBDIR"] = "subdir"
fullPath = os.path.expandvars(inputString)

The problem is that my program is multithreaded and I don't want to globally change os.environ. All I want is the information "fullPath". I'd like to be able to create a local "env" dictionary and pass it to os.path.expandvars as an optional second argument.

I attach a patch for posixpath.py and ntpath.py, though this is untested and is intended to show what I mean in practice.

I've now attached a better patch, which is tested at least on UNIX for now. Decided it was better if the optional argument was a function to replace os.getenv, as then it can be called only on demand and there isn't a need to set up information unnecessarily, or call any code more than once.

File Added: patch

Python has had a way to do this since python 2.4:

>>> from string import Template
>>> fullPath =
Template("$ROOT/${SUBDIR/some/path").substitute(dict(ROOT='/users/geof',
SUBDIR='subdir'))
>>> fullPath
'/users/geof/subdir/some/path'

Thanks, didn't know about that feature.