How to conditionally make python's argparse module require additional arguments

Question

Basic intended usage:

my_framework create Project_title /path/to/project

OR

my_framework create Project_title (ie. use current working directory)

OR

my_framework update (ie. update my_framework rather than creating a new project)

I know I can make name optional by providing it with a default, but, in reality name is not optional provided the user has entered create as the first argument.

Best solution I've come up with is to use a default value for name and then, if the argument name equals its default value, throw an error. But if there's a way to make argparse do this work for me I'd rather learn to do it.

Writing two scripts, my_framework_create and my_framework_update doesn't appeal to me aesthetically.

#!/usr/bin/env python


import argparse
import os
import shutil
from subprocess import call

template_path = "/usr/local/klibs/template"
parser = argparse.ArgumentParser("MY_FRAMEWORK CLI", description='Creates a new MY_FRAMEWORK project or updates MY_FRAMEWORK')
parser.add_argument('action', choices=['create', 'update'], type=str, help='<help text>')
parser.add_argument('name', type=str, help='<help text>')
parser.add_argument('path', default=os.getcwd(), nargs="?", type=str, help='<help text>')
args = parser.parse_args()

if args.action == "create":
    # do the create stuff

if args.action == "update":
    # do the update stuff

You want to use subparsers for this: docs.python.org/2/library/… — Wolph
– Wolph, Commented Jun 26, 2015 at 17:41
I do not fully understand the question. Why should my_framework called with update not require a name? Shouldn't it always require a name, in case a user has two of the frameworks? — Thorsten Kurth
– Thorsten Kurth, Commented Jun 26, 2015 at 17:46
Sorry; update applies to MY_FRAMEWORK not a project created therewith (ie. it basically checks for a new release on github and pulls it if one is found). — Jonline
– Jonline, Commented Jun 26, 2015 at 17:53

Wolph · Accepted Answer · 2015-06-26 18:13:51Z

5

The best way to do this is with a subparser

An example from the docs:

>>> parser = argparse.ArgumentParser()
>>> subparsers = parser.add_subparsers(title='subcommands',
...                                    description='valid subcommands',
...                                    help='additional help')
>>> subparsers.add_parser('foo')
>>> subparsers.add_parser('bar')
>>> parser.parse_args(['-h'])
usage:  [-h] {foo,bar} ...

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

subcommands:
  valid subcommands

  {foo,bar}   additional help

In your case you would have create and update as separate subparsers.

Example:

def create(args):
    # do the create stuff
    print(args)


def update(args):
    # do the update stuff
    print(args)


parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers(title='subcommands',
                                   description='valid subcommands',
                                   help='additional help')

create_parser = subparsers.add_parser('create')
create_parser.add_argument('name', type=str)
create_parser.set_defaults(func=create)

update_parser = subparsers.add_parser('update')
update_parser.set_defaults(func=update)

edited Jun 26, 2015 at 18:13

answered Jun 26, 2015 at 17:44

Wolph

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4 Comments

Jonline Over a year ago

Elegant. And this will demand the subparser's arguments when create is the first argument whilst ignore additional arguments when update is entered?

Wolph Over a year ago

@Jonline: yes, it will :) When update is entered it will go to the second subparser, with create it will go to the first. This should do exactly what you are looking for in a clean way :)

Jonline Over a year ago

Yep I have this running now, charming solution. Two small syntactic bugs in your code (tried to edit but < 6 chars): update_parser subparsers.add_parser('update') needs = and both setdefaults should be snake case.

Wolph Over a year ago

That's what you get for coding in a textarea I suppose. Without syntax highlighting I tend to make mistakes ;)

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