[help] How to decide what to do with command line parameters?

Submitted ⁨⁨1⁩ ⁨year⁩ ago⁩ by ⁨bloopernova@programming.dev⁩ to ⁨programming@programming.dev⁩

This isn’t really language specific, but if it helps I’m using Python. I can get the parameters just fine with the Traitlets module, but I’m still a novice really and figuring out which patterns to use is challenging.

Say you have a bunch of command line parameters. Some are booleans, where their presence means True, absence means False. Other parameters must accept one text string, and others can be used multiple times to build a list of strings.

It feels inefficient/wrong to use a bunch of IF/THEN/ELSE statements to decide what to do with the parameters, and prone to edge case errors. Is there a pattern that would invoke the correct method based on the combination of input parameters?

Examples:

app thing --dry-run --create --name=newname01 --name=newname02 --verbose

app thing --create --name=newname01 --name=newname02 --name=newname03

app job --cancel -i 01 -i 02 -i 03

Thank you, sorry for my noobness.

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nickwitha_k@lemmy.sdf.org ⁨1⁩ ⁨year⁩ ago

As a few others mentioned, the argparse module from the stdlib is the way to go for Python. It takes care of most of this for you in an optimized way. Here’s a minimal example of the above:

#!/bin/env python3

import argparse


# Setup main parser
parser = argparse.ArgumentParser(
    prog="app",
    description="App that manages things and jobs",
)
parser.add_argument(
    "-v",
    "--verbose",
    action="store_true",
)
subparsers = parser.add_subparser(
    help="Sub commands",
    dest="command",
)

# Setup sub-parsers
thing_parser = subparsers.add_parser(
    "thing",
    help="Manage things",
)
# thing_parser args
thing_parser.add_argument(
    "--dry_run",
    action="store_true",
    help="dry run",
)
thing_parser.add_argument(
    "--create",
    action="store_true",
    help="Create thing",
)
thing_parser.add_argument(
    "--name",
    action="store",
    type=str,
    required=True,
    nargs="+",
    help="Thing name",
)

job_parser = subparsers.add_parser(
    "job",
    help="Manage jobs",
)
job_parser.add_argument(
    "--cancel",
    "-c",
    action="store_true",
    help="Cancel job",
)
job_parser.add_argument(
    "--index",
    "-i",
    action="store",
    type=int,
    required=True,
    nargs="+",
    help="Job index",
)


def main():
    args=parser.parse_args()
    if args.verbose:
        print("Verbose mode")
    # Match-Case statement for cleaner logic
    match args.command:
        case "thing":
            # thing logic and function calls here
        case "job":
            # job logic and function calls here
        case _:
            parser.print_usage()

if __name__ == "__main__":
    main()

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bloopernova@programming.dev ⁨1⁩ ⁨year⁩ ago
That’s an awesome answer, thank you very much. It’s much more elegant than my stuff!

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- nickwitha_k@lemmy.sdf.org ⁨1⁩ ⁨year⁩ ago
  You’re very welcome! I’ve spent a lot of time with Python and really think that argparse as of 3.x makes most non-stdlib libraries for parsing are unnecessary. You get a ton of functionally and ability to add end-user documentation as you go, while abstracting away some of the basics like type casting/checking
  
  The addition of Match-Case, while not adding much, functionally, does a LOT for readability and keeping logic clear.
  
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liquid_buffalo@lemmy.world ⁨1⁩ ⁨year⁩ ago
Python has a builtin argparse module. You should use that. It does all of the heavy lifting and logic for you.

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- bloopernova@programming.dev ⁨1⁩ ⁨year⁩ ago
  I was doing it wrong. I was trying to do appname thing --action when I should have been doing appname action --thing. Thank you for commenting, I’m sorry to waste your time.
  
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glad_cat@lemmy.sdf.org ⁨1⁩ ⁨year⁩ ago
It’s never pointless. Command-line arguments are often tricky to get right, especially when your program can do a lot of different things.

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- sj_zero ⁨1⁩ ⁨year⁩ ago
  Strings and human beings are two things that mix to create really tough problems.
Raikin@feddit.de ⁨1⁩ ⁨year⁩ ago
What module are you currently using? I think the built-in module argparse should solve your problem but maybe I’m misunderstanding what you’re asking for.

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- bloopernova@programming.dev ⁨1⁩ ⁨year⁩ ago
  I was doing it wrong. I was trying to do appname thing --action when I should have been doing appname action --thing. Thank you for commenting, I’m sorry to waste your time.
  
  source
  - Raikin@feddit.de ⁨1⁩ ⁨year⁩ ago
    No worries, this ain’t Stackoverflow ;-)
    
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- bloopernova@programming.dev ⁨1⁩ ⁨year⁩ ago
  I already have the arguments parsed, the bit that I’m confused about is where I decide what to do based on those various combinations of arguments.
  
  I’m using Traitlets.
  
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TehPers@beehaw.org ⁨1⁩ ⁨year⁩ ago
Others have already mentioned argparse, which is a pretty great module for argument parsing if you want to use something built-in. If you’re willing to look into other libraries though, I’d recommend taking a look at Click. You should be able to pip install click to use it. It takes advantage of defining the CLI through decorators, and from my experience, works better when you want to have subcommands.

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- bloopernova@programming.dev ⁨1⁩ ⁨year⁩ ago
  Interesting! I’ll check it out.
  
  I’ve replaced the Traitlets based arg parsing with argparse, and the subcommands are working ok so far. I’ll see if Click can add anything, thank you for the recommendation!
  
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sj_zero ⁨1⁩ ⁨year⁩ ago
Seems like a for statement and a case statement could work.

Detect numargs

For a = 0 to numargs

Get core command on arg a (probably read until you hit an equals sign or a space), trim and lcase

Select Case core command

Case "thing"

Do stuff

Case "--name" Get part after equals and do stuff

Case "-i" Get next arg and increment a

End select
- bloopernova@programming.dev ⁨1⁩ ⁨year⁩ ago
  I was doing it wrong. I was trying to do appname thing --action when I should have been doing appname action --thing. Thank you for commenting, I’m sorry to waste your time.
  
  source
  - sj_zero ⁨1⁩ ⁨year⁩ ago
    No worries, it was fun trying to figure out a solution to the problem regardless.