[Python-ideas] argparse - add support for environment variables

[Miki Tebeka]

Greetings,

The usual way of resolving configuration is command line -> environment -> 
default.
Currently argparse supports only command line -> default, I'd like to 
suggest an optional "env" keyword to add_argument that will also resolve 
from environment. (And also optional env dictionary to the ArgumentParser 
__init__ method [or to parse_args], which will default to os.environ).

Example:
         [spam.py]

parser = ArgumentParser()

     parser.add_argument('--spam', env='SPAM', default=7)
     args = parser.parse_args()
     print(args.spam)

     ./spam.py  -> 7
     ./spam.py --spam=12 -> 12
     SPAM=9 ./spam.py -> 9
     SPAM=9 ./spam.py --spam=12 -> 12

What do you think?
--
Miki
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