Quick tip: Using Python’s expandvars() with quoted variables on Windows

I recently encountered an elusive bug, which eventually I tracked down to the fact that environment variables are not replaced in a certain string even though it is set by calling os.path.expandvars(original string).

Turns out, the problem is that expandvars() doesn’t expand variables who are enclosed within a string literal. I couldn’t find it documented anywhere but facts are, this is the case.

Small example:

>>> import os
>>> os.environ['VAR1'] = 'Value1'
>>> s1 = "this is '%VAR1%'"
>>> os.path.expandvars(s1)
"this is '%VAR1%'"
>>> s2 = "this is %VAR1%"
>>> os.path.expandvars(s2)
'this is Value1'

It may be a good idea in some cases, but for me it was quite an annoyance.

So, how to overcome this?

Continue reading


cmd /c doesn’t like quotes

I was quite baffled at first.

This works:

cmd /c "C:\Program Files\7-Zip\7z.exe" C:\Temp\MyFile.7z

But this fails:

cmd /c "C:\Program Files\7-Zip\7z.exe" "C:\Temp\My Other File.7z"
'C:\Program' is not recognized as an internal or external command, operable program or batch file.

Turns out it’s an issue as old as Windows 2000.

The solution is to add another pair of quotes around the entire command:

cmd /c ""C:\Program Files\7-Zip\7z.exe" "C:\Temp\My Other File.7z""

Thank you, Microsoft.