When using the argparse argument fromfile_prefix_chars to obtain command line arguments from a file, it is not possible to make use of bash environment variables within this file. 

Ideally one would be able to `export BAR='/Users/x/Desktop/bar'`, and then give arguments such as:

--foo
$BAR

and have this correctly expanded at read time to --foo '/Users/x/Desktop/bar'.

To my knowledge, this is currently only possible if you give the arguments directly via the command line:
python test.py --foo $BAR

It would be great to be able to use environment variables within files as well!