Environment variables - Expand

[Alex Martelli]

<posted & mailed>

Dips wrote:

> Hi All,
> 
> Is it possible to expand environment variable in Windows using
> one-liner python command?

Python is not really about "one-liners", even though in this
case it might be possible.


> For instance, the following command doesn't expand %WINDIR%
> automatically
> file_name = '%WINDIR%\WIN.INI'
> file = os.open(file,'r')

Some problems here (that backslash is likely to cause problems,
and I doubt you really want to use 'file' in EITHER of the two
spots you use it in the last like), but, let's skip those.

%name% is not Python syntax for "expand this", so of course
it won't.  If you can rather use the Python syntax $(WINDIR)s
for the same functionality, then

filename = filename % os.environ

will do it.


> In actual 'file_name' is fed as command-line argument and so can
> contain any environement variables.

If you're stuck with an "alien" (non-Python) syntax, then the
requirement of parsing and expanding it by a "one-liner" becomes
_particularly_ unreasonable.  You could use A FEW lines with
regular expressions, e.g (warning, untested code):

import re
re_env = re.compile(r'%\w+%')

def expander(mo):
    return os.environ.get(mo.group()[1:-1}, 'UNKNOWN')

filename = re_env.sub(expander, filename)

or you might find some existing program or library routine that
does exactly what you want and call it from Pyhton (e.g. with
os.popen if it's an external program).


Alex