Quickstart

Overview

parsearg is a Python package for writing command-line interfaces ("CLI") that augments (rather than replaces) the standard Python module for writing CLIs, argparse. There is nothing wrong with argparse: It's fine in terms of the functionality that it provides, but it can be clunky to use, especially when a program's structure has subcommands, or nested subcommands (i.e. subcommands that have subcommands). Moreover, because of the imperative nature of argparse, it makes it hard to understand how a program's interface is structured (viz. the program's "view").

parsearg puts a layer on top of argparse that makes writing a CLI easy: You declare your view (i.e. the CLI), with a dict so that the view is a data structure (i.e. pure configuration). The data structure declares the intent of the CLI and you no longer have to instruct argparse on how to put the CLI together: parsearg does that for you. In this respect, parsearg turns argparse on its head, in the sense that it replaces imperative instructions with declarative data.

Usage

Suppose we wish to create a program called quickstart-todos.py to manage the TO-DOs of a set of different users. We want to have subprograms of quickstart-todos.py; for example, we may want to create a user (python quickstart-todos.py create user, say), or we may want to create a TO-DO for a particular user (python quickstart-todos.py create todo, say). We might also want to add optional parameters to each subprogram such as the user's email and phone number, or the TO-DO's due date. An invocation of the program's CLI might look like the following:

$ python quickstart-todos.py create user Bob --email=bob@email.com --phone=+1-212-555-1234
$ python quickstart-todos.py create todo Bob 'taxes' --due-date=2021-05-17

With argparse, the subprogram create would necessitate fiddling with subparsers. With parsearg, the CLI for the above is declared with a dict and parsearg.parser.ParseArg supplants the normal use of argparse.ArgumentParser. Moreover, the callback associated with each subcommand is explicitly linked to its declaration.

import sys
from parsearg import ParseArg

def create_user(args):
    print(f'created user: {args.name!r} (email: {args.email}, phone: {args.phone})')
    
def create_todo(args):
    print(f'created TO-DO for user {args.user!r}: {args.title} (due: {args.due_date})')
    
view = {
    'create|user': {
        'callback':   create_user,
        'name':       {'help': 'create user name', 'action': 'store'},
        '-e|--email': {'help': "create user's email address", 'action': 'store', 'default': ''},
        '-p|--phone': {'help': "create user's phone number", 'action': 'store', 'default': ''},
    },
    'create|todo': {
        'callback':   create_todo,
        'user':       {'help': 'user name', 'action': 'store'},
        'title':      {'help': 'title of TO-DO', 'action': 'store'},
        '-d|--due-date': {'help': 'due date for the TO-DO', 'action': 'store', 'default': None},
    },
}

def main(args):
    # ParseArg takes the place of argparse.ArgumentParser
    parser = ParseArg(d=view)

    # parser.parse_args returns an argparse.Namespace
    ns     = parser.parse_args(args)

    # ns.callback contains the function in the 'callback' key of 'view'
    result = ns.callback(ns)

if __name__ == "__main__":
    args = sys.argv[1:] if len(sys.argv) > 1 else []
    
    main(' '.join(args))

A fully-worked version of this TO-DO example is presented in the docs. The output of the above is:

$ python quickstart-todos.py create user Bob --email=bob@email.com --phone=212-555-1234
created user: 'Bob' (email: bob@email.com, phone: 212-555-1234)

$ python quickstart-todos.py create todo Bob 'taxes' --due-date=2021-05-17
created TO-DO for user 'Bob': taxes (due: 2021-05-17)

Because parsearg is built on top of argparse, all the usual features are available, such as the extensive help features (essentially making the CLI self-documenting):

$ python quickstart-todos.py --help
usage: quickstart-todos.py [-h] {create} ...

positional arguments:
  {create}

optional arguments:
  -h, --help  show this help message and exit

$ python quickstart-todos.py create --help
usage: quickstart-todos.py create [-h] {todo,user} ...

positional arguments:
  {todo,user}

optional arguments:
  -h, --help   show this help message and exit

$ python quickstart-todos.py create user --help
usage: quickstart-todos.py create user [-h] [-e EMAIL] [-p PHONE] name

positional arguments:
  name                  create user name

optional arguments:
  -h, --help            show this help message and exit
  -e EMAIL, --email EMAIL
                        create user's email address
  -p PHONE, --phone PHONE
                        create user's phone number
$ python quickstart-todos.py create todo --help
usage: quickstart-todos.py create todo [-h] [-d DUE_DATE] user title

positional arguments:
  user                  user name
  title                 title of to-do

optional arguments:
  -h, --help            show this help message and exit
  -d DUE_DATE, --due-date DUE_DATE
                        due date for the to-do