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parsearg turns argparse on its head the declarative way

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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.


Suppose we wish to create a program called to manage the TO-DOs of a set of different users. We want to have subprograms of; for example, we may want to create a user (python create user, say), or we may want to create a TO-DO for a particular user (python 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 create user Bob --phone=+1-212-555-1234
$ python 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: {!r} (email: {}, 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 create user Bob --phone=212-555-1234
created user: 'Bob' (email:, phone: 212-555-1234)

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

positional arguments:

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

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

positional arguments:

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

$ python create user --help
usage: 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 create todo --help
usage: 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

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