(Those too after a long talk to a colleague at work, this time #py/argparse)

Cool things about argparse:¹

• parser.add_argument('--two-words') would automatically map to args.two_words (_ vs -)!
• One can provide complex types!² For files, two options.
  • The first one allows to set file permissions etc., but it opens them and returns the handle to you, which you may not want.
  • pathlib.Path() works as expected, and even automagically parses string paths from args into the Path!
    • Additionally we can then establish that we’re working with Paths from the very beginning, getting rid of the str or Path ambiguity.

    • “Be strict and clear from the very beginning, then you don’t have to deal Path or str”

  • Sample of both from official documentation:

    parser.add_argument('a', type=argparse.FileType('w', encoding='latin-1'))
    parser.add_argument('b', type=pathlib.Path)
    

• You can get defalut values from os.environ()! Then you can also run it as

  WHATVEER_VALUE=234 python3 file.py
  

A nice structure for it all is:

1. if __name__ == '__main__': runs a function like main() getting rid of the scope issues
2. Parsing is done my a separate function, that returns the Namespace:

   def parse_args() -> argparse.Namespace:
       parser = argparse.ArgumentParser()
       parser.add_argument('--input-directory' ..)
       return parser.parse_args()
   

3. Then in main() we use it like args = parse_args(); if args.input_directory == ... This is nice also because then we don’t have to deal with an arparse object in main, just its results.

Also, in general, CLI programs have arguments like program --arg-one, not program --arg_one. I write the latter one because I still feel I’m in a python world, but Python would parse such dashed arguments into classic ones (see above). TODO look for some best practices for CLI programs, including Python ones, POSIX etc etc etc.